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, 2015		34,242,255
Balance, beginning of the period at Dec. 31, 2015	$ 1,300,059		$ 1,261,174	$ 39,615	$ (730)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	85,385			85,385
Other comprehensive income, net of income tax	(2,725)				(2,725)
Accrual of dividends on common stock	(29,672)			(29,672)
Proceeds from issuance of common stock for stockholder reinvestment program (in shares)		96,382
Repurchase of common stock under the terms of the repurchase plan (in shares)		(1,145,250)
Stock Repurchased and Retired During Period, Value	(50,772)		(50,772)
Amortization of stock-based compensation related to restricted stock grants, net of shares surrendered	3,401		3,401
Excess tax benefit on stock-based compensation	34		34
Balance, end of the period (in shares) at Dec. 31, 2016		33,193,387
Balance, end 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	74,324			74,324
Other comprehensive income, net of income tax	3,579				3,579
Accrual of dividends on common stock	(58,247)			(58,247)
Proceeds from issuance of common stock for stockholder reinvestment program (in shares)		86,397
Repurchase of common stock under the terms of the repurchase plan (in shares)		(25,000)
Stock Repurchased and Retired During Period, Value	(1,400)		(1,400)
Amortization of stock-based compensation related to restricted stock grants, net of shares surrendered	3,045		3,045
Balance, end of the period (in shares) at Sep. 30, 2017		33,254,784
Balance, end of the period at Sep. 30, 2017	$ 1,327,011		$ 1,215,482	$ 111,405	$ 124