Earnings Per Share - Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic Earnings per Common Share
Net income attributable to Company	$ 30,617	$ 27,845	$ 14,336	$ 19,182	$ 12,897	$ 23,268	$ 14,435	$ 16,951	$ 91,980	$ 67,551	$ 78,013
Less: Preferred stock dividends									(15,660)	(12,900)	(10,990)
NET INCOME ATTRIBUTABLE TO COMPANY'S COMMON STOCKHOLDERS	$ 24,632	$ 24,620	$ 11,111	$ 15,957	$ 9,672	$ 20,043	$ 11,210	$ 13,726	$ 76,320	$ 54,651	$ 67,023
Weighted average common shares outstanding-basic (in shares)	111,871	111,886	111,863	111,721	109,911	109,569	109,489	109,402	111,836	109,594	108,399
Basic Earnings per Common Share (in dollars per share)	$ 0.22	$ 0.22	$ 0.10	$ 0.14	$ 0.09	$ 0.18	$ 0.10	$ 0.13	$ 0.68	$ 0.50	$ 0.62
Diluted Earnings per Common Share:
Net income attributable to Company	$ 30,617	$ 27,845	$ 14,336	$ 19,182	$ 12,897	$ 23,268	$ 14,435	$ 16,951	$ 91,980	$ 67,551	$ 78,013
Less: Preferred stock dividends									(15,660)	(12,900)	(10,990)
Add back: Interest expense on convertible notes for the period, net of tax									9,158	0	0
Net income attributable to Company's common stockholders									$ 85,478	$ 54,651	$ 67,023
Weighted average common shares outstanding-basic (in shares)	111,871	111,886	111,863	111,721	109,911	109,569	109,489	109,402	111,836	109,594	108,399
Net effect of assumed convertible notes conversion to common shares (in shares)									18,507	0	0
Diluted weighted average common shares outstanding (in shares)	131,565	131,580	111,863	126,602	109,911	109,569	109,489	109,402	130,343	109,594	108,399
Diluted Earnings per Common Share (in dollars per share)	$ 0.21	$ 0.21	$ 0.10	$ 0.14	$ 0.09	$ 0.18	$ 0.10	$ 0.13	$ 0.66	$ 0.50	$ 0.62