Unaudited Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Paid-in Capital	Retained Earnings Substantially Restricted	Accumulated Other Comprehensive (Loss) Income
Balance, beginning, shares at Dec. 31, 2017		78,006,956
Balance, beginning at Dec. 31, 2017	$ 1,199,057	$ 780	$ 797,997	$ 402,259	$ (1,979)
Net income	132,279			132,279
Other comprehensive income (loss)	(27,738)				(27,738)
Cash dividends declared	(63,517)			(63,517)
Stock issuances under stock incentive plans, shares		81,269
Stock issuances under stock incentive plans, value	0	$ 1	(1)
Stock issued in connection with acquisitions, shares		6,432,868
Stock issued in connection with acquisitions, value	250,807	$ 64	250,743
Stock-based compensation and related taxes	1,724		1,724
Balance, ending, shares at Sep. 30, 2018		84,521,093
Balance, ending at Sep. 30, 2018	1,492,612	$ 845	1,050,463	471,021	(29,717)
Balance, beginning, shares at Jun. 30, 2018		84,516,650
Balance, beginning at Jun. 30, 2018	1,473,992	$ 845	1,049,724	443,705	(20,282)
Net income	49,336			49,336
Other comprehensive income (loss)	(9,435)				(9,435)
Cash dividends declared	(22,020)			(22,020)
Stock issuances under stock incentive plans, shares		4,443
Stock issuances under stock incentive plans, value	0	$ 0	0
Stock-based compensation and related taxes	739		739
Balance, ending, shares at Sep. 30, 2018		84,521,093
Balance, ending at Sep. 30, 2018	$ 1,492,612	$ 845	1,050,463	471,021	(29,717)
Balance, beginning, shares at Dec. 31, 2018	84,521,692	84,521,692
Balance, beginning at Dec. 31, 2018	$ 1,515,854	$ 845	1,051,253	473,183	(9,427)
Net income	153,134			153,134
Other comprehensive income (loss)	57,522				57,522
Cash dividends declared	(72,260)			(72,260)
Stock issuances under stock incentive plans, shares		139,309
Stock issuances under stock incentive plans, value	0	$ 2	(2)
Stock issued in connection with acquisitions, shares		7,519,617
Stock issued in connection with acquisitions, value	316,538	$ 75	316,463
Stock-based compensation and related taxes	$ 8,071		8,071
Balance, ending, shares at Sep. 30, 2019	92,180,618	92,180,618
Balance, ending at Sep. 30, 2019	$ 1,953,401	$ 922	1,375,785	528,599	48,095
Balance, beginning, shares at Jun. 30, 2019		86,637,394
Balance, beginning at Jun. 30, 2019	1,687,376	$ 866	1,139,289	503,773	43,448
Net income	51,610			51,610
Other comprehensive income (loss)	4,647				4,647
Cash dividends declared	(26,784)			(26,784)
Stock issuances under stock incentive plans, shares		69,948
Stock issuances under stock incentive plans, value	0	$ 1	(1)
Stock issued in connection with acquisitions, shares		5,473,276
Stock issued in connection with acquisitions, value	229,385	$ 55	229,330
Stock-based compensation and related taxes	$ 7,167		7,167
Balance, ending, shares at Sep. 30, 2019	92,180,618	92,180,618
Balance, ending at Sep. 30, 2019	$ 1,953,401	$ 922	$ 1,375,785	528,599	$ 48,095
Cumulative Effect of New Accounting Principle in Period of Adoption	$ (25,458)			$ (25,458)