Condensed Financial Information - Parent Company Only Cash Flows (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows From Operating Activities:
Net Income	$ 171,753,000	$ 161,625,000	$ 149,502,000
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities [Abstract]
Amortization of financing costs	845,000	617,000	582,000
Stock-based compensation	5,209,000	6,556,000	5,938,000
Excess tax benefits from stock-based compensation	0	(964,000)	(201,000)
Increase in other assets	29,227,000	(29,940,000)	(22,987,000)
Loss on redemption of trust preferred securities	0	0	5,626,000
Increase (decrease) in other liabilities and payable to non-bank subsidiaries	(31,412,000)	4,427,000	4,928,000
Total adjustments	87,073,000	23,785,000	28,042,000
Net cash provided by operating activities	258,826,000	185,410,000	177,544,000
Cash Flows From Investing Activities
Net cash used in investing activities	(1,166,441,000)	(1,000,416,000)	(818,027,000)
Cash Flows From Financing Activities:
Repayments of long-term debt	(115,153,000)	(236,640,000)	(540,079,000)
Additions to long-term debt	223,251,000	215,884,000	347,778,000
Net proceeds from issuance of common stock	8,538,000	16,167,000	10,607,000
Excess tax benefits from stock-based compensation	0	964,000	201,000
Dividends paid	(80,368,000)	(69,382,000)	(65,361,000)
Acquisition of treasury stock	0	(18,545,000)	(50,000,000)
Net cash provided by financing activities	897,143,000	832,649,000	635,901,000
Net Increase (Decrease) in Cash and Cash Equivalents	(10,472,000)	17,643,000	(4,582,000)
Parent
Cash Flows From Operating Activities:
Net Income	171,753,000	161,625,000	149,502,000
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities [Abstract]
Amortization of financing costs	845,000	0	0
Stock-based compensation	4,740,000	6,556,000	5,938,000
Excess tax benefits from stock-based compensation	0	(964,000)	(201,000)
Increase in other assets	(17,882,000)	(16,585,000)	2,806,000
Equity in undistributed net income of subsidiaries	(128,298,000)	(65,340,000)	(58,884,000)
Loss on redemption of trust preferred securities	0	0	5,626,000
Increase (decrease) in other liabilities and payable to non-bank subsidiaries	31,241,000	(5,928,000)	106,490,000
Total adjustments	(109,354,000)	(82,261,000)	61,775,000
Net cash provided by operating activities	62,399,000	79,364,000	211,277,000
Cash Flows From Investing Activities
Net cash used in investing activities	0	0	0
Cash Flows From Financing Activities:
Repayments of long-term debt	(100,000,000)	0	(254,640,000)
Additions to long-term debt	123,251,000	0	147,779,000
Net proceeds from issuance of common stock	9,007,000	16,167,000	10,607,000
Excess tax benefits from stock-based compensation	0	964,000	201,000
Dividends paid	(80,368,000)	(69,382,000)	(65,361,000)
Acquisition of treasury stock	0	(18,545,000)	(50,000,000)
Net cash provided by financing activities	(48,110,000)	(70,796,000)	(211,414,000)
Net Increase (Decrease) in Cash and Cash Equivalents	14,289,000	8,568,000	(137,000)
Cash and Cash Equivalents at Beginning of Year	8,568,000	0	137,000
Cash and Cash Equivalents at End of Year	$ 22,857,000	$ 8,568,000	$ 0