CONSOLIDATED STATEMENT OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - 9 months ended Sep. 30, 2016 - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Treasury Stock [Member]	Total
Balance at Dec. 31, 2015	$ 44,443	$ 528,015	$ 566,591	$ 19,235	$ (17,637)	$ 1,140,647
Balance (in shares) at Dec. 31, 2015	43,774,860
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income			77,420			77,420
Other comprehensive income, net of tax				46,856		46,856
Cash dividends declared:
Common, $0.94 per share			(41,498)			(41,498)
Common stock issued under employee stock plan, including tax benefits of $2,229	$ 414	7,142				7,556
Common stock issued under employee stock plan, including tax benefits of $2,229 (in shares)	413,509
Stock-based compensation		3,392				3,392
Treasury stock issued to benefit plan, net		2,000			4,209	6,209
Treasury stock issued to benefit plan, net (in shares)	169,027
Balance at Sep. 30, 2016	$ 44,857	$ 540,549	$ 602,513	$ 66,091	$ (13,428)	$ 1,240,582
Balance (in shares) at Sep. 30, 2016	44,357,396