Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Paid-in Capital	Retained Earnings Substantially Restricted	Accumulated Other Comprehensive Income (Loss)
Balance, shares at Dec. 31, 2015		76,086,288
Balance at Dec. 31, 2015	$ 1,076,650	$ 761	$ 736,368	$ 337,532	$ 1,989
Net income	121,131			121,131
Other comprehensive income (loss)	(9,371)				(9,371)
Cash dividends declared	(84,284)			(84,284)
Stock issued in connection with acquisitions, shares		349,545
Stock issued in connection with acquisitions, amount	10,465	$ 3	10,462
Stock issuances under stock incentive plans, shares		89,569
Stock issuances under stock incentive plans, value	0	$ 1	(1)
Stock-based compensation and related taxes	2,278		2,278
Balance, shares at Dec. 31, 2016		76,525,402
Balance at Dec. 31, 2016	1,116,869	$ 765	749,107	374,379	(7,382)
Net income	116,377			116,377
Other comprehensive income (loss)	5,754			351	5,403
Cash dividends declared	(88,848)			(88,848)
Stock issued in connection with acquisitions, shares		1,381,661
Stock issued in connection with acquisitions, amount	46,673	$ 14	46,659
Stock issuances under stock incentive plans, shares		99,893
Stock issuances under stock incentive plans, value	0	$ 1	(1)
Stock-based compensation and related taxes	$ 2,232		2,232
Balance, shares at Dec. 31, 2017	78,006,956	78,006,956
Balance at Dec. 31, 2017	$ 1,199,057	$ 780	797,997	402,259	(1,979)
Net income	181,878			181,878
Other comprehensive income (loss)	(7,448)				(7,448)
Cash dividends declared	(110,954)			(110,954)
Stock issued in connection with acquisitions, shares		6,432,868
Stock issued in connection with acquisitions, amount	250,807	$ 64	250,743
Stock issuances under stock incentive plans, shares		81,868
Stock issuances under stock incentive plans, value	0	$ 1	(1)
Stock-based compensation and related taxes	$ 2,514		2,514
Balance, shares at Dec. 31, 2018	84,521,692	84,521,692
Balance at Dec. 31, 2018	$ 1,515,854	$ 845	$ 1,051,253	$ 473,183	$ (9,427)