Selected Quarterly Financial Data Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Interest Income (Expense), Net [Abstract]
Total interest income	$ 302,739	$ 292,642	$ 288,514	$ 277,775	$ 284,597	$ 260,966	$ 259,544	$ 247,409	$ 1,161,670	$ 1,052,516	$ 1,015,912
Interest expense	(40,974)	(32,325)	(26,182)	(21,275)	(21,641)	(19,276)	(17,071)	(14,957)	(120,756)	(72,945)	(54,621)
Net interest income	261,765	260,317	262,332	256,500	262,956	241,690	242,473	232,452	1,040,914	979,571	961,291
Provision for credit losses	12,000	11,500	17,500	4,000	6,406	15,119	11,499	(24,728)	45,000	57,752	65,729
Interest Income (Expense), after Provision for Loan Loss	249,765	248,817	244,832	252,500	256,550	226,571	230,974	207,724	995,914	921,819	895,562
FDIC loss sharing expense, net									0	0	(8,917)
Noninterest Income [Abstract]
Gain (loss) on sale of securities	786	826	253	6,311	(3,329)	1,236	1,651	(99)	8,176	(541)	9,485
Noninterest Income, Other	32,740	36,086	39,385	32,248	30,124	30,146	33,631	35,213
Total noninterest income	33,526	36,912	39,638	38,559	26,795	31,382	35,282	35,114	148,635	128,573	112,475
Foreclosed Assets (Income) Expense, Net	311	257	61	122	475	(2,191)	157	(143)
Noninterest Expense [Abstract]
Amortization	(970)	(800)	0	0	(16,085)	(1,450)	(1,700)	(500)	(22,506)	(14,240)	(16,517)
Acquisition, integration and reorganization costs	(128,576)	(127,610)	(126,510)	(127,517)	(127,258)	(114,901)	(116,164)	(115,901)	(1,770)	(19,735)	(200)
Total noninterest expense	(129,235)	(128,153)	(126,449)	(127,395)	(142,868)	(118,542)	(117,707)	(116,544)	(511,232)	(495,661)	(450,101)
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent [Abstract]
Earnings from continuing operations before taxes	154,056	157,576	158,021	163,664	140,477	139,411	148,549	126,294	633,317	554,731	557,936
Income Tax Expense (Benefit)	(39,015)	(41,289)	42,286	45,388	$ (56,440)	$ (37,945)	$ (54,902)	$ (47,626)	(167,978)	(196,913)	(205,770)
Net earnings	$ 115,041	$ 116,287	$ 115,735	$ 118,276					$ 465,339	$ 357,818	$ 352,166
Earnings Per Share, Diluted	$ 0.93	$ 0.94	$ 0.92	$ 0.93	$ 0.66	$ 0.84	$ 0.77	$ 0.65	$ 3.72	$ 2.91	$ 2.90
Net earnings from continuing operations					$ 84,037	$ 101,466	$ 93,647	$ 78,668	$ 465,339	$ 357,818
Earnings Per Share, Basic [Abstract]
Basic and diluted earnings per share (usd per share)			0.92	0.93	$ 0.66	$ 0.84	$ 0.77	$ 0.65
Dividends declared per share (usd per share)	$ 0.60	$ 0.60	$ 0.60	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 2.30	$ 2.00	$ 2.00