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	59,133			59,133
Comprehensive income, accumulated other comprehensive income (loss)	18,424				18,424
Comprehensive income, total	77,557
Cash dividends declared	(30,560)			(30,560)
Stock issuances under stock incentive plans, shares		85,292
Stock issuances under stock incentive plans, value	0	$ 1	(1)
Stock-based compensation and related taxes	1,012		1,012
Balance, ending, shares at Jun. 30, 2016		76,171,580
Balance, ending at Jun. 30, 2016	$ 1,124,659	$ 762	737,379	366,105	20,413
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	64,942			64,942
Comprehensive income, accumulated other comprehensive income (loss)	13,081				13,081
Comprehensive income, total	78,023
Cash dividends declared	(32,550)			(32,550)
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		94,827
Stock issuances under stock incentive plans, value	0	$ 1	(1)
Stock-based compensation and related taxes	$ 942		942
Balance, ending, shares at Jun. 30, 2017	78,001,890	78,001,890
Balance, ending at Jun. 30, 2017	$ 1,209,957	$ 780	$ 796,707	$ 406,771	$ 5,699