Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Activities
Net income	$ 29,508	$ 29,614	$ 29,335	$ 27,670	$ 28,054	$ 29,294	$ 28,677	$ 26,730	$ 116,127	$ 112,755	$ 95,644
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on sale of investments									(19)	188	299
Excess tax (benefits) deficiencies from stock-based compensation									(102)	(138)	223
Net cash provided by operating activities									171,466	182,734	334,095
Investing Activities
Sales of available-for-sale securities									136,777	169,372	181,971
Maturities, prepayments and calls of available-for-sale securities									663,828	628,238	1,682,363
Net addition of premises and equipment									(18,224)	(14,389)	(8,977)
Net sale (purchase) of non-marketable equity securities									27,770	801	583
Net cash (used in) provided by investing activities									(676,571)	158,893	149,237
Financing Activities
Net increase in other borrowed funds									(566)	(933)	(1,502)
Cash dividends paid									(79,456)	(50,944)	(44,232)
Excess tax benefits (deficiencies) from stock-based compensation									102	138	(223)
Stock-based compensation activity									17	785	4,326
Net cash used in financing activities									255,949	(54,875)	(514,715)
Net (decrease) increase in cash and cash equivalents									(249,156)	286,752	(31,383)
Cash and cash equivalents at beginning of period				442,409				155,657	442,409	155,657	187,040
Cash and cash equivalents at end of period	193,253				442,409				193,253	442,409	155,657
Parent Company
Operating Activities
Net income									116,127	112,755	95,644
Adjustments to reconcile net income to net cash provided by operating activities:
Subsidiary income in excess of dividends distributed									(13,782)	(39,451)	(37,129)
Loss on sale of investments									0	0	3,248
Excess tax (benefits) deficiencies from stock-based compensation									(102)	(138)	223
Net change in other assets and other liabilities									307	140	2,575
Net cash provided by operating activities									102,550	73,306	64,561
Investing Activities
Sales of available-for-sale securities									0	0	23,990
Maturities, prepayments and calls of available-for-sale securities									0	0	2,571
Changes in investment securities and other stock - intercompany									0	0	(946)
Net addition of premises and equipment									(1,405)	(179)	(603)
Net sale (purchase) of non-marketable equity securities									22	(667)	0
Equity contribution to subsidiaries									(28,457)	(18,115)	(11,336)
Net cash (used in) provided by investing activities									(29,840)	(18,961)	13,676
Financing Activities
Net increase in other borrowed funds									143	143	144
Cash dividends paid									(79,456)	(50,944)	(44,232)
Excess tax benefits (deficiencies) from stock-based compensation									102	138	(223)
Stock-based compensation activity									17	785	4,326
Net cash used in financing activities									(79,194)	(49,878)	(39,985)
Net (decrease) increase in cash and cash equivalents									(6,484)	4,467	38,252
Cash and cash equivalents at beginning of period				$ 55,146				$ 50,679	55,146	50,679	12,427
Cash and cash equivalents at end of period	$ 48,662				$ 55,146				$ 48,662	$ 55,146	$ 50,679