Selected Quarterly Financial Data Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest Income (Expense), Net [Abstract]
Total interest income	$ 293,593	$ 307,208	$ 314,533	$ 304,559	$ 302,739	$ 292,642	$ 288,514	$ 277,775	$ 1,219,893	$ 1,161,670	$ 1,052,516
Interest expense	(46,974)	(54,972)	(53,635)	(49,683)	(40,974)	(32,325)	(26,182)	(21,275)	(205,264)	(120,756)	(72,945)
Net interest income	246,619	252,236	260,898	254,876	261,765	260,317	262,332	256,500	1,014,629	1,040,914	979,571
Provision for credit losses	(3,000)	(7,000)	(8,000)	(4,000)	(12,000)	(11,500)	(17,500)	(4,000)	(22,000)	(45,000)	(57,752)
Interest Income (Expense), after Provision for Loan Loss	243,619	245,236	252,898	250,876	249,765	248,817	244,832	252,500	992,629	995,914	921,819
Noninterest Income [Abstract]
Gain (loss) on sale of securities	184	908	22,192	2,161	786	826	253	6,311	25,445	8,176	(541)
Noninterest Income, Other	26,992	32,521	28,701	28,903	32,740	36,086	39,385	32,248
Total noninterest income	27,176	33,429	50,893	31,064	33,526	36,912	39,638	38,559	142,562	148,635	128,573
Foreclosed Assets (Income) Expense, Net	3,446	(8)	146	(29)	311	257	61	122
Noninterest Expense [Abstract]
Acquisition, integration and reorganization costs	269	0	0	(618)	(970)	(800)	0	0
Other expense	127,443	126,801	125,573	125,640	128,576	127,610	126,510	127,517	30,913	39,741	37,997
Total noninterest expense	(123,728)	(126,809)	(125,427)	(126,287)	(129,235)	(128,153)	(126,449)	(127,395)	(502,251)	(511,232)	(495,661)
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent [Abstract]
Earnings from continuing operations before taxes	147,067	151,856	178,364	155,653	154,056	157,576	158,021	163,664	632,940	633,317	554,731
Income Tax Expense (Benefit)	(29,186)	(41,830)	(50,239)	(43,049)	(39,015)	(41,289)	(42,286)	(45,388)	(164,304)	(167,978)	(196,913)
Net earnings	$ 117,881	$ 110,026	$ 128,125	$ 112,604					468,636	465,339	$ 357,818
Net earnings from continuing operations					$ 115,041	$ 116,287	$ 115,735	$ 118,276	$ 468,636	$ 465,339
Earnings Per Share, Basic [Abstract]
Basic and diluted earnings per share (usd per share)	$ 0.98	$ 0.92	$ 1.07	$ 0.92	$ 0.93	$ 0.94	$ 0.92	$ 0.93
Dividends declared per share (usd per share)	$ 0.60	$ 0.60	$ 0.60	$ 0.60	$ 0.60	$ 0.60	$ 0.60	$ 0.50	$ 2.40	$ 2.30	$ 2.00