Condensed Financial Information (Parent Company Only) - Condensed Statements of Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions
Salaries and benefits	$ 122,751	$ 108,684	$ 101,451
Interest expense	27,934	17,661	18,060
Acquisition related expenses	27,156	2,821	795
Earnings before income tax benefit	156,722	145,259	130,457
Income tax benefit	50,201	49,623	43,662
Net income	106,521	95,636	86,795
Parent Company
Condensed Financial Statements, Captions
Dividends from subsidiaries	150,000	140,000	70,000
Other interest income	100	0	0
Other income, primarily management fees from subsidiaries	18,000	15,100	13,200
Total income	168,100	155,100	83,200
Salaries and benefits	21,800	18,800	16,300
Interest expense	4,700	4,100	3,800
Acquisition related expenses	25,300	1,500	800
Other operating expenses, net	13,000	10,400	9,900
Total expenses	64,800	34,800	30,800
Earnings before income tax benefit	103,300	120,300	52,400
Income tax benefit	(14,200)	(7,700)	(7,000)
Income before undistributed earnings of subsidiaries	117,500	128,000	59,400
Undistributed earnings of subsidiaries	(11,000)	(32,400)	27,400
Net income	$ 106,500	$ 95,600	$ 86,800