Employee Benefit Plans - Awards Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Options [Member]
Summary of options issued and outstanding
Balance at the beginning of the period (in shares)	1,170,593
Options granted (in shares)	0	0	0
Options exercised (in shares)	(331,639)
Options forfeited/expired (in shares)	0
Balance at the end of the period (in shares)	838,954	1,170,593
Weighted Average Exercise Price
Balance at the beginning of the period (in dollars per share)	$ 18.43
Options granted (in dollars per share)	0.00
Options exercised (in dollars per share)	21.02
Options forfeited (in dollars per share)	0.00
Balance at the end of the period (in dollars per share)	$ 17.41	$ 18.43
Number of Exercisable Options (in shares)	838,954	1,145,356
Performance Stock Options [Member]
Summary of options issued and outstanding
Balance at the beginning of the period (in shares)	221,409
Options granted (in shares)	0	0	0
Options exercised (in shares)	(30,508)
Options forfeited/expired (in shares)	0
Balance at the end of the period (in shares)	190,901	221,409
Weighted Average Exercise Price
Balance at the beginning of the period (in dollars per share)	$ 17.89
Options granted (in dollars per share)	0.00
Options exercised (in dollars per share)	18.45
Options forfeited (in dollars per share)	0.00
Balance at the end of the period (in dollars per share)	$ 17.80	$ 17.89
Number of Exercisable Options (in shares)	190,901	221,409
Restricted Stock Awards [Member]
Restricted Stock Award and Restricted Stock Unit Activity
Nonvested at the beginning of the period (in shares)	58,858
Granted (in shares)	50,225
Vested (in shares)	(58,858)
Forfeited (in shares)	0
Nonvested at the end of the period (in shares)	50,225	58,858
Weighted Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share)	$ 25.57
Granted (in dollars per share)	37.93
Vested (in dollars per share)	25.57
Forfeited (in dollars per share)	0.00
Nonvested at the end of the period (in dollars per share)	$ 37.93	$ 25.57
Restricted Stock Units (RSUs) [Member]
Restricted Stock Award and Restricted Stock Unit Activity
Nonvested at the beginning of the period (in shares)	945,509
Granted (in shares)	245,735
Vested (in shares)	(289,176)
Forfeited (in shares)	(47,449)
Nonvested at the end of the period (in shares)	854,619	945,509
Weighted Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share)	$ 24.01
Granted (in dollars per share)	41.37
Vested (in dollars per share)	22.74
Forfeited (in dollars per share)	23.35
Nonvested at the end of the period (in dollars per share)	29.67	$ 24.01
Performance Restricted Stock Units [Member]
Weighted Average Exercise Price
Weighted Average Grant Date Fair Value (in dollars per share)	$ 53.74	$ 25.62	$ 28.31
Restricted Stock Award and Restricted Stock Unit Activity
Nonvested at the beginning of the period (in shares)	609,435
Granted (in shares)	315,896	155,000
Vested (in shares)	(318,467)
Forfeited (in shares)	0
Nonvested at the end of the period (in shares)	606,864	609,435
Weighted Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share)	$ 26.22
Granted (in dollars per share)	53.74	$ 25.17
Vested (in dollars per share)	25.17
Forfeited (in dollars per share)	0.00
Nonvested at the end of the period (in dollars per share)	$ 33.80	$ 26.22
Excluded due to nonperformance (in shares)	426,670