CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock [Member]	Common Stock and Paid in Capital [Member]	Retained Earnings (Accumulated Deficit) [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance, beginning of the period (in shares) at Dec. 31, 2016		33,193,387
Balance, beginning of the period at Dec. 31, 2016	$ 1,305,710		$ 1,213,837	$ 95,328	$ (3,455)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	60,776			60,776
Other comprehensive loss, net of income tax	(786)				(786)
Reclassification of stranded tax effects from AOCI to retained earnings	0			795	(795)
Accrual of dividends on common stock	(66,364)			(66,364)
Repurchase of common stock		(545,166)
Proceeds from issuance of common stock for stockholder reinvestment program (in shares)		78,264
Repurchase of common stock, value	(31,045)		(31,045)
Amortization of stock-based compensation related to restricted stock grants, net of shares surrendered	4,335		4,335
Balance, end of the period (in shares) at Dec. 31, 2017		32,726,485
Balance, end of the period at Dec. 31, 2017	1,272,626		1,187,127	90,535	(5,036)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of reclassification of the instrument-specific credit risk portion of junior subordinated debentures fair value adjustments and reclassification of equity securities from available-for-sale	0			(28,203)	28,203
Net income	61,214			61,214
Other comprehensive loss, net of income tax	(28,298)				(28,298)
Accrual of dividends on common stock	(39,061)			(39,061)
Repurchase of common stock		(269,711)
Proceeds from issuance of common stock for stockholder reinvestment program (in shares)		(51,078)
Repurchase of common stock, value	(15,359)		(15,359)
Amortization of stock-based compensation related to restricted stock grants, net of shares surrendered	1,888		1,888
Balance, end of the period (in shares) at Jun. 30, 2018		32,405,696
Balance, end of the period at Jun. 30, 2018	$ 1,253,010		$ 1,173,656	$ 84,485	$ (5,131)