CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2015	$ 524,938	$ 20,781	$ 108,747	$ 58,433	$ 336,977
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through affiliate's stock plans	4,277	182	4,095
Repurchase of common stock	(383)	(17)	(366)
Increase resulting from stock-based compensation expense associated with affiliate's stock plans allocated to the Company	24		24
Other comprehensive income	11,484			11,484
Net income	14,654				14,654
Dividends paid to public stockholders	(1,691)				(1,691)
Dividends paid to affiliate	(2,237)				(2,237)
Balance at Mar. 31, 2016	551,066	20,946	112,500	69,917	347,703
Balance at Dec. 31, 2016	553,342	21,223	119,054	46,081	366,984
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through affiliate's stock plans	1,953	76	1,877
Repurchase of common stock	(1,758)	(64)	(1,694)
Increase resulting from stock-based compensation expense associated with affiliate's stock plans allocated to the Company	7		7
Other comprehensive income	8,354			8,354
Net income	6,804				6,804
Dividends paid to public stockholders	(1,956)				(1,956)
Dividends paid to affiliate	(2,472)				(2,472)
Balance at Mar. 31, 2017	$ 564,274	$ 21,235	$ 119,244	$ 54,435	$ 369,360