Condensed Statements of Cash Flows (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Activities
Net income	$ 112,755	$ 95,644	$ 75,516
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on sale of investments	188	299	0
Excess tax (benefits) deficiencies from stock-based compensation	(138)	223	8
Net cash provided by operating activities	182,734	334,095	183,793
Investing Activities
Maturities, prepayments and calls of available-for-sale securities	628,238	1,682,363	2,041,416
Net addition of premises and equipment	(14,389)	(8,977)	(10,730)
Net sale of non-marketable equity securities	801	583	888
Net cash provided by (used in) investing activities	158,893	149,237	(579,646)
Financing Activities
Net increase in other borrowed funds	(933)	(1,502)	180
Cash dividends paid	(50,944)	(44,232)	(47,472)
Excess tax benefits (deficiencies) from stock-based compensation	138	(223)	(8)
Stock-based compensation activity	785	4,326	81
Net cash used in financing activities	(54,875)	(514,715)	454,861
Net increase (decrease) in cash and cash equivalents	286,752	(31,383)	59,008
Cash and cash equivalents at beginning of period	155,657	187,040	128,032
Cash and cash equivalents at end of period	442,409	155,657	187,040
Parent Company
Operating Activities
Net income	112,755	95,644	75,516
Adjustments to reconcile net income to net cash provided by operating activities:
Subsidiary income (in excess of) less than dividends distributed	(39,451)	(37,129)	(1,040)
Loss on sale of investments	0	3,248	0
Excess tax (benefits) deficiencies from stock-based compensation	(138)	223	8
Net change in other assets and other liabilities	140	2,575	3,684
Net cash provided by operating activities	73,306	64,561	78,168
Investing Activities
Proceeds from Sale, Maturity and Collection of Investments	0	23,990	0
Maturities, prepayments and calls of available-for-sale securities	0	2,571	787
Changes in investment securities and other stock - intercompany	0	(946)	(19,183)
Net addition of premises and equipment	(179)	(603)	(2,927)
Net sale of non-marketable equity securities	(667)	0	0
Equity contribution to subsidiaries	(18,115)	(11,336)	(28,500)
Net cash provided by (used in) investing activities	(18,961)	13,676	(49,823)
Financing Activities
Net increase in other borrowed funds	143	144	143
Cash dividends paid	(50,944)	(44,232)	(47,472)
Excess tax benefits (deficiencies) from stock-based compensation	138	(223)	(8)
Stock-based compensation activity	785	4,326	81
Net cash used in financing activities	(49,878)	(39,985)	(47,256)
Net increase (decrease) in cash and cash equivalents	4,467	38,252	(18,911)
Cash and cash equivalents at beginning of period	50,679	12,427	31,338
Cash and cash equivalents at end of period	$ 55,146	$ 50,679	$ 12,427