Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Activities
Net income	$ 31,041	$ 30,957	$ 30,451	$ 28,682	$ 29,508	$ 29,614	$ 29,335	$ 27,670	$ 121,131	$ 116,127	$ 112,755
Adjustments to reconcile net income to net cash provided by operating activities:
Excess tax benefits from stock-based compensation									(8)	(102)	(138)
Net cash provided by operating activities									180,402	171,466	182,734
Investing Activities
Net sale (purchase) of non-marketable equity securities									6,017	27,770	801
Net cash used in by investing activities									(407,964)	(676,571)	158,893
Financing Activities
Net increase in other borrowed funds									(2,019)	(566)	(933)
Cash dividends paid									(84,040)	(79,456)	(50,944)
Excess tax benefits from stock-based compensation									8	102	138
Stock-based compensation activity									(600)	17	785
Net cash used in financing activities									186,850	255,949	(54,875)
Net increase (decrease) in cash and cash equivalents									(40,712)	(249,156)	286,752
Cash and cash equivalents at beginning of period				193,253				442,409	193,253	442,409	155,657
Cash and cash equivalents at end of period	152,541				193,253				152,541	193,253	442,409
Parent Company
Operating Activities
Net income									121,131	116,127	112,755
Adjustments to reconcile net income to net cash provided by operating activities:
Subsidiary income in excess of dividends distributed									(19,548)	(13,782)	(39,451)
Excess tax benefits from stock-based compensation									(8)	(102)	(138)
Net change in other assets and other liabilities									507	307	140
Net cash provided by operating activities									102,082	102,550	73,306
Investing Activities
Net decrease (increase) of premises and equipment									771	(1,405)	(179)
Net sale (purchase) of non-marketable equity securities									55	22	(667)
Equity contribution to subsidiaries									(3,475)	(28,457)	(18,115)
Net cash used in by investing activities									(2,649)	(29,840)	(18,961)
Financing Activities
Net increase in other borrowed funds									143	143	143
Cash dividends paid									(84,040)	(79,456)	(50,944)
Excess tax benefits from stock-based compensation									8	102	138
Stock-based compensation activity									(600)	17	785
Net cash used in financing activities									(84,489)	(79,194)	(49,878)
Net increase (decrease) in cash and cash equivalents									14,944	(6,484)	4,467
Cash and cash equivalents at beginning of period				$ 48,662				$ 55,146	48,662	55,146	50,679
Cash and cash equivalents at end of period	$ 63,606				$ 48,662				$ 63,606	$ 48,662	$ 55,146