Earnings per Share (EPS) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 44,479	$ 40,821	$ 37,193	$ 34,838	$ 28,866	$ 25,070	$ 19,555	$ 15,006	$ 157,331	$ 88,497	$ 65,413
Net income allocable to Common									$ 157,331	$ 88,497	$ 12,706
Basic earnings (loss) per share (in dollars per share)	$ 0.49	$ 0.45	$ 0.41	$ 0.39	$ 0.34	$ 0.30	$ 0.23	$ 0.18	$ 1.74	$ 1.05	$ 0.90
Diluted earnings (loss) per share (in dollars per share)	$ 0.48	$ 0.44	$ 0.41	$ 0.38	$ 0.33	$ 0.29	$ 0.23	$ 0.18	$ 1.72	$ 1.03	$ 0.70
Basic weighted average shares outstanding (in shares)	90,454	90,418	90,344	90,185	86,134	83,640	83,276	82,864	90,351	83,986	14,044
Dilutive effect of nonvested shares (in shares)									1,387	1,616	4,059
Diluted weighted average shares outstanding (in shares)	91,918	91,841	91,674	91,514	87,950	85,028	84,706	84,696	91,738	85,602	18,103
Class A Common Shares
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income allocable to Common											$ 52,707