Annual report pursuant to Section 13 and 15(d)

Benefit Plans - Summary of Restricted Stock Unit and Stock Award Activity (Details)

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Benefit Plans - Summary of Restricted Stock Unit and Stock Award Activity (Details) - Restricted Stock Units (RSUs) and Performance Shares - $ / shares
shares in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Shares      
Nonvested as of beginning of period (in shares) 29,893 16,088 11,575
Granted (in shares) 11,721 18,523 8,961
Vested (in shares) (8,842) (4,212) (3,464)
Forfeited (in shares) (1,449) (506) (984)
Nonvested as of end of period (in shares) 31,323 29,893 16,088
Grant Date Fair Value Per Share      
Nonvested as of beginning of period (in dollars per share) $ 4.03 $ 3.73 $ 3.47
Granted (in dollars per share) 5.35 4.21 3.92
Vested (in dollars per share) 3.92 3.68 3.44
Forfeited (in dollars per share) 4.42 3.75 3.52
Nonvested as of end of period (in dollars per share) $ 4.54 $ 4.03 $ 3.73