Annual report pursuant to Section 13 and 15(d)

Quarterly Financial Data -- Unaudited (Details)

v3.6.0.2
Quarterly Financial Data -- Unaudited (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Jun. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]                        
Total revenue $ 1,302,998 $ 1,277,646 $ 1,235,566 $ 1,201,010 $ 1,196,146 $ 1,169,712 $ 1,123,210 $ 1,080,990   $ 5,017,220 $ 4,570,058 $ 4,181,095
Cost of services (551,323) (488,659) (486,317) (467,028) (470,523) (440,808) (525,463) (406,370)        
Income from operations 329,560 392,179 362,156 348,234 293,869 351,584 219,429 313,806   1,432,129 1,178,688 1,119,670
Net income $ 204,627 $ 193,901 $ 174,965 $ 172,440 $ 134,633 $ 166,550 $ 102,849 $ 105,692 $ 347,405 $ 745,933 $ 509,724 $ 493,241
Net income per common share--basic (in dollars per share) $ 0.04 $ 0.04 $ 0.04 $ 0.03 $ 0.03 $ 0.03 $ 0.02 $ 0.02 $ 0.07 $ 0.15 $ 0.09 $ 0.09
Net income per common share--diluted (in dollars per share) $ 0.04 $ 0.04 $ 0.04 $ 0.03 $ 0.03 $ 0.03 $ 0.02 $ 0.02 $ 0.07 $ 0.15 $ 0.09 $ 0.08