Loans and Allowances for Credit Losses, Activity in Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Allowance for loan losses [Roll Forward]
Beginning balance	$ 230,682	$ 246,159
Provision for loan losses	(5,401)	1,804
Loans charged off	(2,890)	(2,153)
Recoveries	1,576	2,900
Ending balance	223,967	248,710
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	3,734	11,244
Provision for off-balance sheet credit losses	401	(1,804)
Ending balance	4,135	9,440
Total provision for credit losses	(5,000)	0
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment			$ 17,157,786	$ 16,965,550	$ 16,784,290
Individually measured for impairment, recorded investment			180,064	187,874	207,616
Total			17,337,850	17,153,424	16,991,906
Related allowance [Abstract]
Collectively measured for impairment, related allowance			190,630	199,634	217,886
Individually measured for impairment, related allowance			13,362	8,831	3,497
Total	230,682	246,159	223,967	230,682	248,710
Commercial
Allowance for loan losses [Roll Forward]
Beginning balance	124,269	140,213
Provision for loan losses	(3,111)	(3,355)
Loans charged off	(1,563)	(424)
Recoveries	488	1,182
Ending balance	120,083	137,616
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	3,644	11,063
Provision for off-balance sheet credit losses	383	(1,775)
Ending balance	4,027	9,288
Total provision for credit losses	(2,728)	(5,130)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment			10,788,207	10,596,672	10,170,285
Individually measured for impairment, recorded investment			131,460	137,303	156,825
Total			10,919,667	10,733,975	10,327,110
Related allowance [Abstract]
Collectively measured for impairment, related allowance			106,721	115,438	134,164
Individually measured for impairment, related allowance			13,362	8,831	3,452
Total	124,269	140,213	120,083	124,269	137,616
Commercial real estate
Allowance for loan losses [Roll Forward]
Beginning balance	56,621	50,749
Provision for loan losses	266	6,859
Loans charged off	0	0
Recoveries	183	735
Ending balance	57,070	58,343
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	45	123
Provision for off-balance sheet credit losses	(1)	(17)
Ending balance	44	106
Total provision for credit losses	265	6,842
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment			3,504,312	3,477,132	3,866,588
Individually measured for impairment, recorded investment			2,470	2,855	4,475
Total			3,506,782	3,479,987	3,871,063
Related allowance [Abstract]
Collectively measured for impairment, related allowance			57,070	56,621	58,343
Individually measured for impairment, related allowance			0	0	0
Total	56,621	50,749	57,070	56,621	58,343
Residential mortgage
Allowance for loan losses [Roll Forward]
Beginning balance	18,451	18,224
Provision for loan losses	(162)	(39)
Loans charged off	(100)	(236)
Recoveries	242	228
Ending balance	18,431	18,177
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	43	50
Provision for off-balance sheet credit losses	19	(10)
Ending balance	62	40
Total provision for credit losses	(143)	(49)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment			1,899,975	1,926,239	1,900,193
Individually measured for impairment, recorded investment			45,794	47,447	46,081
Total			1,945,769	1,973,686	1,946,274
Related allowance [Abstract]
Collectively measured for impairment, related allowance			18,431	18,451	18,132
Individually measured for impairment, related allowance			0	0	45
Total	18,451	18,224	18,431	18,451	18,177
Personal
Allowance for loan losses [Roll Forward]
Beginning balance	9,124	8,773
Provision for loan losses	(152)	(788)
Loans charged off	(1,227)	(1,493)
Recoveries	663	755
Ending balance	8,408	7,247
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	2	8
Provision for off-balance sheet credit losses	0	(2)
Ending balance	2	6
Total provision for credit losses	(152)	(790)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment			965,292	965,507	847,224
Individually measured for impairment, recorded investment			340	269	235
Total			965,632	965,776	847,459
Related allowance [Abstract]
Collectively measured for impairment, related allowance			8,408	9,124	7,247
Individually measured for impairment, related allowance			0	0	0
Total	9,124	8,773	8,408	9,124	7,247
Specific Allowance [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	208,465
Ending balance	203,992	221,383
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment			17,157,786	16,965,550	16,784,290
Individually measured for impairment, recorded investment			180,064	187,874	207,616
Total			17,337,850	17,153,424	16,991,906
Related allowance [Abstract]
Collectively measured for impairment, related allowance			190,630	199,634	217,886
Individually measured for impairment, related allowance			13,362	8,831	3,497
Total	208,465	221,383	203,992	208,465	221,383
Nonspecific Allowance [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	22,217	28,200
Provision for loan losses	(2,242)	(873)
Loans charged off	0	0
Recoveries	0	0
Ending balance	19,975	27,327
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	0	0
Provision for off-balance sheet credit losses	0	0
Ending balance	0	0
Total provision for credit losses	(2,242)	(873)
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	$ 22,217	$ 28,200	$ 19,975	$ 22,217	$ 27,327