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	82,943			82,943
Other comprehensive income (loss)	(18,303)				(18,303)
Cash dividends declared	(41,497)			(41,497)
Stock issuances under stock incentive plans, shares		76,826
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	985		985
Balance, ending, shares at Jun. 30, 2018		84,516,650
Balance, ending at Jun. 30, 2018	1,473,992	$ 845	1,049,724	443,705	(20,282)
Balance, beginning, shares at Mar. 31, 2018		84,511,472
Balance, beginning at Mar. 31, 2018	1,454,024	$ 845	1,048,860	421,342	(17,023)
Net income	44,384			44,384
Other comprehensive income (loss)	(3,259)				(3,259)
Cash dividends declared	(22,021)			(22,021)
Stock issuances under stock incentive plans, shares		5,178
Stock issuances under stock incentive plans, value	0
Stock-based compensation and related taxes	864		864
Balance, ending, shares at Jun. 30, 2018		84,516,650
Balance, ending at Jun. 30, 2018	$ 1,473,992	$ 845	1,049,724	443,705	(20,282)
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	101,524			101,524
Other comprehensive income (loss)	52,875				52,875
Cash dividends declared	(45,476)			(45,476)
Stock issuances under stock incentive plans, shares		69,361
Stock issuances under stock incentive plans, value	0	$ 1	(1)
Stock issued in connection with acquisitions, shares		2,046,341
Stock issued in connection with acquisitions, value	87,153	$ 20	87,133
Stock-based compensation and related taxes	$ 904		904
Balance, ending, shares at Jun. 30, 2019	86,637,394	86,637,394
Balance, ending at Jun. 30, 2019	$ 1,687,376	$ 866	1,139,289	503,773	43,448
Balance, beginning, shares at Mar. 31, 2019		84,588,199
Balance, beginning at Mar. 31, 2019	1,550,850	$ 846	1,051,299	474,818	23,887
Net income	52,392			52,392
Other comprehensive income (loss)	19,561				19,561
Cash dividends declared	(23,437)			(23,437)
Stock issuances under stock incentive plans, shares		2,854
Stock issuances under stock incentive plans, value	0	$ 0	0
Stock issued in connection with acquisitions, shares		2,046,341
Stock issued in connection with acquisitions, value	87,153	$ 20	87,133
Stock-based compensation and related taxes	$ 857		857
Balance, ending, shares at Jun. 30, 2019	86,637,394	86,637,394
Balance, ending at Jun. 30, 2019	$ 1,687,376	$ 866	$ 1,139,289	$ 503,773	$ 43,448