Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Paid-in Capital	Retained Earnings Substantially Restricted	Accumulated Other Comprehensive Income
Balance, shares at Dec. 31, 2012		71,937,222
Balance at Dec. 31, 2012	$ 900,949	$ 719	$ 641,737	$ 210,531	$ 47,962
Net income	95,644			95,644
Comprehensive income (loss), accumulated other comprehensive income	(38,317)				(38,317)
Comprehensive income, total	57,327
Cash dividends declared	(44,232)			(44,232)
Stock issuances under stock incentive plans, shares		292,942
Stock issuances under stock incentive plans, amount	4,507	$ 3	4,504
Stock issued in connection with acquisitions, shares		2,143,132
Stock issued in connection with acquisitions, amount	45,033	$ 22	45,011
Stock-based compensation and related taxes	(334)		(334)
Balance, shares at Dec. 31, 2013		74,373,296
Balance at Dec. 31, 2013	963,250	$ 744	690,918	261,943	9,645
Net income	112,755			112,755
Comprehensive income (loss), accumulated other comprehensive income	8,099				8,099
Comprehensive income, total	120,854
Cash dividends declared	(73,501)			(73,501)
Stock issuances under stock incentive plans, shares		97,064
Stock issuances under stock incentive plans, amount	784	$ 1	783
Stock issued in connection with acquisitions, shares		555,732
Stock issued in connection with acquisitions, amount	15,127	$ 5	15,122
Stock-based compensation and related taxes	$ 1,533		1,533
Balance, shares at Dec. 31, 2014	75,026,092	75,026,092
Balance at Dec. 31, 2014	$ 1,028,047	$ 750	708,356	301,197	17,744
Net income	116,127			116,127
Comprehensive income (loss), accumulated other comprehensive income	(15,755)				(15,755)
Comprehensive income, total	100,372
Cash dividends declared	(79,792)			(79,792)
Stock issuances under stock incentive plans, shares		62,346
Stock issuances under stock incentive plans, amount	17	$ 1	16
Stock issued in connection with acquisitions, shares		997,850
Stock issued in connection with acquisitions, amount	25,939	$ 10	25,929
Stock-based compensation and related taxes	$ 2,067		2,067
Balance, shares at Dec. 31, 2015	76,086,288	76,086,288
Balance at Dec. 31, 2015	$ 1,076,650	$ 761	$ 736,368	$ 337,532	$ 1,989