Loans and Allowances for Credit Losses, Activity in Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018
Allowance for loan losses [Roll Forward]
Beginning balance	$ 207,457	$ 230,682
Provision for loan losses	7,969	(5,401)
Loans charged off	(11,775)	(2,890)
Recoveries	1,689	1,576
Ending balance	205,340	223,967
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	1,790	3,734
Provision for off-balance sheet credit losses	31	401
Ending balance	1,821	4,135
Total provision for credit losses	8,000	(5,000)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment			$ 21,606,403	$ 21,493,483
Individually measured for impairment, recorded investment			152,577	163,247
Total			21,758,980	21,656,730
Related allowance [Abstract]
Collectively measured for impairment, related allowance			182,619	180,957
Individually measured for impairment, related allowance			3,543	8,732
Total	207,457	230,682	205,340	207,457
Commercial [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	102,226	124,269
Provision for loan losses	11,108	(3,111)
Loans charged off	(10,468)	(1,563)
Recoveries	711	488
Ending balance	103,577	120,083
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	1,655	3,644
Provision for off-balance sheet credit losses	70	383
Ending balance	1,725	4,027
Total provision for credit losses	11,178	(2,728)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment			13,871,617	13,536,237
Individually measured for impairment, recorded investment			90,358	99,841
Total			13,961,975	13,636,078
Related allowance [Abstract]
Collectively measured for impairment, related allowance			100,034	93,494
Individually measured for impairment, related allowance			3,543	8,732
Total	103,577	124,269	103,577	102,226
Commercial real estate [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	60,026	56,621
Provision for loan losses	(2,004)	266
Loans charged off	0	0
Recoveries	112	183
Ending balance	58,134	57,070
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	52	45
Provision for off-balance sheet credit losses	(4)	(1)
Ending balance	48	44
Total provision for credit losses	(2,008)	265
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment			4,579,143	4,743,192
Individually measured for impairment, recorded investment			21,508	21,621
Total			4,600,651	4,764,813
Related allowance [Abstract]
Collectively measured for impairment, related allowance			58,134	60,026
Individually measured for impairment, related allowance			0	0
Total	60,026	56,621	58,134	60,026
Residential mortgage [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	17,964	18,451
Provision for loan losses	(2,408)	(162)
Loans charged off	(42)	(100)
Recoveries	154	242
Ending balance	15,668	18,431
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	52	43
Provision for off-balance sheet credit losses	(5)	19
Ending balance	47	62
Total provision for credit losses	(2,413)	(143)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment			2,152,211	2,188,478
Individually measured for impairment, recorded investment			40,409	41,555
Total			2,192,620	2,230,033
Related allowance [Abstract]
Collectively measured for impairment, related allowance			15,668	17,964
Individually measured for impairment, related allowance			0	0
Total	15,668	18,451	15,668	17,964
Personal [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	9,473	9,124
Provision for loan losses	(137)	(152)
Loans charged off	(1,265)	(1,227)
Recoveries	712	663
Ending balance	8,783	8,408
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	31	2
Provision for off-balance sheet credit losses	(30)	0
Ending balance	1	2
Total provision for credit losses	(167)	(152)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment			1,003,432	1,025,576
Individually measured for impairment, recorded investment			302	230
Total			1,003,734	1,025,806
Related allowance [Abstract]
Collectively measured for impairment, related allowance			8,783	9,473
Individually measured for impairment, related allowance			0	0
Total	9,473	9,124	8,783	9,473
Specific Allowance [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	189,689
Ending balance	186,162
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment			21,606,403	21,493,483
Individually measured for impairment, recorded investment			152,577	163,247
Total			21,758,980	21,656,730
Related allowance [Abstract]
Collectively measured for impairment, related allowance			182,619	180,957
Individually measured for impairment, related allowance			3,543	8,732
Total	186,162		186,162	189,689
Nonspecific Allowance [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	17,768	22,217
Provision for loan losses	1,410	(2,242)
Loans charged off	0	0
Recoveries	0	0
Ending balance	19,178	19,975
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	1,410	(2,242)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment			0	0
Individually measured for impairment, recorded investment			0	0
Total			0	0
Related allowance [Abstract]
Collectively measured for impairment, related allowance			0	0
Individually measured for impairment, related allowance			0	0
Total	$ 19,178	$ 22,217	$ 19,178	$ 17,768