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 Income (Loss)
Balance, beginning, shares at Dec. 31, 2015		76,086,288
Balance, beginning at Dec. 31, 2015	$ 1,076,650	$ 761	$ 736,368	$ 337,532	$ 1,989
Net income	90,090			90,090
Comprehensive income, accumulated other comprehensive income (loss)	14,849				14,849
Comprehensive income, total	104,939
Cash dividends declared	(45,909)			(45,909)
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	1,634		1,634
Balance, ending, shares at Sep. 30, 2016		76,525,402
Balance, ending at Sep. 30, 2016	$ 1,147,779	$ 765	748,463	381,713	16,838
Balance, beginning, shares at Dec. 31, 2016	76,525,402	76,525,402
Balance, beginning at Dec. 31, 2016	$ 1,116,869	$ 765	749,107	374,379	(7,382)
Net income	101,421			101,421
Comprehensive income, accumulated other comprehensive income (loss)	12,049				12,049
Comprehensive income, total	113,470
Cash dividends declared	(72,427)			(72,427)
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	$ 1,616		1,616
Balance, ending, shares at Sep. 30, 2017	78,006,956	78,006,956
Balance, ending at Sep. 30, 2017	$ 1,206,201	$ 780	$ 797,381	$ 403,373	$ 4,667