Earnings per Share (EPS) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 72,118	$ 52,258	$ 138,716	$ 100,209
Less: dividends declared	0	0	0	0
Net income	$ 72,118	$ 52,258	$ 138,716	$ 100,209	$ 222,606
Basic earnings per share (in dollars per share)	$ 0.79	$ 0.57	$ 1.52	$ 1.10
Diluted earnings per share (in dollars per share)	$ 0.77	$ 0.57	$ 1.49	$ 1.09
Basic weighted average shares outstanding (in shares)	91,381	90,912	91,320	90,848
Dilutive effect of nonvested shares (in shares)	1,781	1,226	1,773	1,151
Diluted weighted average shares outstanding (in shares)	93,162	92,138	93,093	91,999