Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Capital Surplus	Retained Earnings	Accumulated Other Comprehensive (Loss) Income	Treasury Stock
Beginning Balance at Dec. 31, 2014	$ 1,643,758	$ 55,057	$ 894,602	$ 963,911	$ 11,006	$ (280,818)
Total comprehensive income	50,832			63,979	(13,147)
Dividends	(22,327)			(22,327)
Purchase of treasury stock	(5,379)					(5,379)
Issuance of equity awards	460		(5,509)			5,969
Recognition of equity-based compensation	5,779		5,779
Net tax benefit related to equity compensation plans	664		664
Sale of treasury stock	503		306			197
Exercise of stock options	3,029		1,488			1,541
Common stock issuance for acquisition	179,737		112,635			67,102
Ending Balance at Jun. 30, 2015	1,857,056	55,057	1,009,965	1,005,563	(2,141)	(211,388)
Beginning Balance at Dec. 31, 2015	1,893,694	55,057	1,019,889	1,033,990	(3,718)	(211,524)
Total comprehensive income	132,555			73,542	59,013
Dividends	(24,252)			(24,252)
Purchase of treasury stock	(13,581)					(13,581)
Issuance of equity awards	430		(4,457)			4,887
Recognition of equity-based compensation	5,200		5,200
Net tax benefit related to equity compensation plans	250		250
Sale of treasury stock	569		260			309
Exercise of stock options	7,867		2,053			5,814
Ending Balance at Jun. 30, 2016	$ 2,002,732	$ 55,057	$ 1,023,195	$ 1,083,280	$ 55,295	$ (214,095)