Earnings per Share (EPS) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 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	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income	$ 62,686	$ 59,711	$ 52,258	$ 47,951	$ 44,479	$ 40,821	$ 37,193	$ 34,838	$ 222,606	$ 157,331	$ 88,497
Net income allocable to Common									$ 222,606	$ 157,331	$ 88,497
Basic earnings per share (in dollars per share)	$ 0.69	$ 0.66	$ 0.57	$ 0.53	$ 0.49	$ 0.45	$ 0.41	$ 0.39	$ 2.45	$ 1.74	$ 1.05
Diluted earnings per share (in dollars per share)	$ 0.68	$ 0.65	$ 0.57	$ 0.52	$ 0.48	$ 0.44	$ 0.41	$ 0.38	$ 2.41	$ 1.72	$ 1.03
Basic weighted average shares outstanding (in shares)	90,991	90,961	90,912	90,785	90,454	90,418	90,344	90,185	90,913	90,351	83,986
Dilutive effect of nonvested shares (in shares)									1,332	1,387	1,616
Diluted weighted average shares outstanding (in shares)	92,577	92,399	92,138	91,859	91,918	91,841	91,674	91,514	92,245	91,738	85,602