Condensed Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Common Shares	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income	Total
Balances at Dec. 31, 2013	$ 6	$ 627,647	$ 66,636	$ 7,201	$ 701,490
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net income			35,819		35,819
Other comprehensive income				7,084	7,084
Dividends			(69,998)		(69,998)
Compensation expense under stock incentive plans		312			312
Balances at Sep. 30, 2014	6	627,959	32,457	14,285	674,707
Balances at Dec. 31, 2014	6	628,236	41,323	18,356	687,921
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net income			40,827		40,827
Other comprehensive income				(9,470)	(9,470)
Dividends			(13,898)		(13,898)
Exercise of stock options and related excess tax benefits		(758)			(758)
Compensation expense under stock incentive plans		2,794			2,794
Balances at Sep. 30, 2015	$ 6	$ 630,272	$ 68,252	$ 8,886	$ 707,416