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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 52,258	$ 37,193	$ 100,209	$ 72,031
Net income available to common shareholders	$ 52,258	$ 37,193	$ 100,209	$ 72,031	$ 157,331
Basic earnings per share (in dollars per share)	$ 0.57	$ 0.41	$ 1.10	$ 0.80
Diluted earnings per share (in dollars per share)	$ 0.57	$ 0.41	$ 1.09	$ 0.79
Basic weighted average shares outstanding (in shares)	90,912	90,344	90,848	90,265
Dilutive effect of nonvested shares (in shares)	1,226	1,330	1,151	1,329
Diluted weighted average shares outstanding (in shares)	92,138	91,674	91,999	91,594