Loans and Allowances for Credit Losses, Activity in Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Allowance for loan losses [Roll Forward]
Beginning balance	$ 223,967	$ 248,710	$ 230,682	$ 246,159
Provision for loan losses	1,702	3,009	(3,699)	4,813
Loans charged off	(15,105)	(2,872)	(17,995)	(5,025)
Recoveries	4,578	1,214	6,154	4,114
Ending balance	215,142	250,061	215,142	250,061
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	4,135	9,440	3,734	11,244
Provision for off-balance sheet credit losses	(1,702)	(3,009)	(1,301)	(4,813)
Ending balance	2,433	6,431	2,433	6,431
Total provision for credit losses	0	0	(5,000)	0
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					$ 17,838,039	$ 16,965,550	$ 16,938,206
Individually measured for impairment, recorded investment					165,657	187,874	245,439
Total					18,003,696	17,153,424	17,183,645
Related allowance [Abstract]
Collectively measured for impairment, related allowance					184,470	199,634	212,994
Individually measured for impairment, related allowance					15,200	8,831	9,693
Total	223,967	248,710	230,682	246,159	215,142	230,682	250,061
Commercial
Allowance for loan losses [Roll Forward]
Beginning balance	120,083	137,616	124,269	140,213
Provision for loan losses	7,116	1,546	4,005	(1,809)
Loans charged off	(13,775)	(1,703)	(15,338)	(2,127)
Recoveries	298	283	786	1,465
Ending balance	113,722	137,742	113,722	137,742
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	4,027	9,288	3,644	11,063
Provision for off-balance sheet credit losses	(1,666)	(2,987)	(1,283)	(4,762)
Ending balance	2,361	6,301	2,361	6,301
Total provision for credit losses	5,450	(1,441)	2,722	(6,571)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					11,228,061	10,596,672	10,440,798
Individually measured for impairment, recorded investment					120,978	137,303	197,157
Total					11,349,039	10,733,975	10,637,955
Related allowance [Abstract]
Collectively measured for impairment, related allowance					98,522	115,438	128,049
Individually measured for impairment, related allowance					15,200	8,831	9,693
Total	120,083	137,616	124,269	140,213	113,722	124,269	137,742
Commercial real estate
Allowance for loan losses [Roll Forward]
Beginning balance	57,070	58,343	56,621	50,749
Provision for loan losses	(1,409)	105	(1,143)	6,964
Loans charged off	0	(76)	0	(76)
Recoveries	3,097	208	3,280	943
Ending balance	58,758	58,580	58,758	58,580
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	44	106	45	123
Provision for off-balance sheet credit losses	(27)	(22)	(28)	(39)
Ending balance	17	84	17	84
Total provision for credit losses	(1,436)	83	(1,171)	6,925
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					3,710,224	3,477,132	3,684,817
Individually measured for impairment, recorded investment					1,996	2,855	3,775
Total					3,712,220	3,479,987	3,688,592
Related allowance [Abstract]
Collectively measured for impairment, related allowance					58,758	56,621	58,580
Individually measured for impairment, related allowance					0	0	0
Total	57,070	58,343	56,621	50,749	58,758	56,621	58,580
Residential mortgage
Allowance for loan losses [Roll Forward]
Beginning balance	18,431	18,177	18,451	18,224
Provision for loan losses	(257)	(47)	(419)	(86)
Loans charged off	(135)	(40)	(235)	(276)
Recoveries	505	169	747	397
Ending balance	18,544	18,259	18,544	18,259
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	62	40	43	50
Provision for off-balance sheet credit losses	(9)	(2)	10	(12)
Ending balance	53	38	53	38
Total provision for credit losses	(266)	(49)	(409)	(98)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					1,899,907	1,926,239	1,894,963
Individually measured for impairment, recorded investment					42,343	47,447	44,235
Total					1,942,250	1,973,686	1,939,198
Related allowance [Abstract]
Collectively measured for impairment, related allowance					18,544	18,451	18,259
Individually measured for impairment, related allowance					0	0	0
Total	18,431	18,177	18,451	18,224	18,544	18,451	18,259
Personal
Allowance for loan losses [Roll Forward]
Beginning balance	8,408	7,247	9,124	8,773
Provision for loan losses	755	1,358	603	570
Loans charged off	(1,195)	(1,053)	(2,422)	(2,546)
Recoveries	678	554	1,341	1,309
Ending balance	8,646	8,106	8,646	8,106
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	2	6	2	8
Provision for off-balance sheet credit losses	0	2	0	0
Ending balance	2	8	2	8
Total provision for credit losses	755	1,360	603	570
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					999,847	965,507	917,628
Individually measured for impairment, recorded investment					340	269	272
Total					1,000,187	965,776	917,900
Related allowance [Abstract]
Collectively measured for impairment, related allowance					8,646	9,124	8,106
Individually measured for impairment, related allowance					0	0	0
Total	8,408	7,247	9,124	8,773	8,646	9,124	8,106
Specific Allowance [Member]
Allowance for loan losses [Roll Forward]
Beginning balance			208,465
Ending balance	199,670	222,687	199,670	222,687
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					17,838,039	16,965,550	16,938,206
Individually measured for impairment, recorded investment					165,657	187,874	245,439
Total					18,003,696	17,153,424	17,183,645
Related allowance [Abstract]
Collectively measured for impairment, related allowance					184,470	199,634	212,994
Individually measured for impairment, related allowance					15,200	8,831	9,693
Total	199,670	222,687	208,465	222,687	199,670	208,465	222,687
Nonspecific Allowance [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	19,975	27,327	22,217	28,200
Provision for loan losses	(4,503)	47	(6,745)	(826)
Loans charged off	0	0	0	0
Recoveries	0	0	0	0
Ending balance	15,472	27,374	15,472	27,374
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	0	0	0	0
Provision for off-balance sheet credit losses	0	0	0	0
Ending balance	0	0	0	0
Total provision for credit losses	(4,503)	47	(6,745)	(826)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					0	0	0
Individually measured for impairment, recorded investment					0	0	0
Total					0	0	0
Related allowance [Abstract]
Collectively measured for impairment, related allowance					0	0	0
Individually measured for impairment, related allowance					0	0	0
Total	$ 19,975	$ 27,327	$ 22,217	$ 28,200	$ 15,472	$ 22,217	$ 27,374