Loans and Allowances for Credit Losses, Activity in Allowance for Credit Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Allowance for loan losses [Roll Forward]
Beginning balance	$ 246,159	$ 225,524	$ 189,056
Provision for loan losses	510	55,467	33,519
Loans charged off	(25,599)	(42,588)	(15,171)
Recoveries	9,612	7,756	18,120
Ending balance	230,682	246,159	225,524
Accrual for off-balance sheet credit losses [Roll Forward]
Beginning balance	11,244	1,711	1,230
Provision for off-balance sheet credit risk	(7,510)	9,533	481
Ending balance	3,734	11,244	1,711
Total provision for credit losses	(7,000)	65,000	34,000
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment				$ 16,965,550	$ 16,758,676
Individually measured for impairment, recorded investment				187,874	230,984
Total				17,153,424	16,989,660
Related allowance [Abstract]
Collectively measured for impairment, related allowance				199,634	217,116
Individually measured for impairment, related allowance				8,831	843
Total measured for impairment, related allowance	246,159	225,524	189,056	230,682	246,159
Commercial [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	140,213	130,334	90,875
Provision for loan losses	(595)	43,980	43,464
Loans charged off	(19,810)	(35,828)	(6,734)
Recoveries	4,461	1,727	2,729
Ending balance	124,269	140,213	130,334
Accrual for off-balance sheet credit losses [Roll Forward]
Beginning balance	11,063	1,506	475
Provision for off-balance sheet credit risk	(7,419)	9,557	1,031
Ending balance	3,644	11,063	1,506
Total provision for credit losses	(8,014)	53,537	44,495
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment				10,596,672	10,211,871
Individually measured for impairment, recorded investment				137,303	178,953
Total				10,733,975	10,390,824
Related allowance [Abstract]
Collectively measured for impairment, related allowance				115,438	139,416
Individually measured for impairment, related allowance				8,831	797
Total measured for impairment, related allowance	140,213	130,334	90,875	124,269	140,213
Commercial Real Estate [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	50,749	41,391	42,445
Provision for loan losses	4,008	8,075	(11,189)
Loans charged off	(76)	0	(944)
Recoveries	1,940	1,283	11,079
Ending balance	56,621	50,749	41,391
Accrual for off-balance sheet credit losses [Roll Forward]
Beginning balance	123	153	707
Provision for off-balance sheet credit risk	(78)	(30)	(554)
Ending balance	45	123	153
Total provision for credit losses	3,930	8,045	(11,743)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment				3,477,132	3,803,525
Individually measured for impairment, recorded investment				2,855	5,521
Total				3,479,987	3,809,046
Related allowance [Abstract]
Collectively measured for impairment, related allowance				56,621	50,749
Individually measured for impairment, related allowance				0	0
Total measured for impairment, related allowance	50,749	41,391	42,445	56,621	50,749
Residential Mortgage [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	18,224	19,509	23,458
Provision for loan losses	116	(1,972)	(3,004)
Loans charged off	(649)	(1,312)	(2,205)
Recoveries	760	1,999	1,260
Ending balance	18,451	18,224	19,509
Accrual for off-balance sheet credit losses [Roll Forward]
Beginning balance	50	30	28
Provision for off-balance sheet credit risk	(7)	20	2
Ending balance	43	50	30
Total provision for credit losses	109	(1,952)	(3,002)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment				1,926,239	1,903,612
Individually measured for impairment, recorded investment				47,447	46,220
Total				1,973,686	1,949,832
Related allowance [Abstract]
Collectively measured for impairment, related allowance				18,451	18,178
Individually measured for impairment, related allowance				0	46
Total measured for impairment, related allowance	18,224	19,509	23,458	18,451	18,224
Personal [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	8,773	4,164	4,233
Provision for loan losses	2,964	7,310	2,167
Loans charged off	(5,064)	(5,448)	(5,288)
Recoveries	2,451	2,747	3,052
Ending balance	9,124	8,773	4,164
Accrual for off-balance sheet credit losses [Roll Forward]
Beginning balance	8	22	20
Provision for off-balance sheet credit risk	(6)	(14)	2
Ending balance	2	8	22
Total provision for credit losses	2,958	7,296	2,169
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment				965,507	839,668
Individually measured for impairment, recorded investment				269	290
Total				965,776	839,958
Related allowance [Abstract]
Collectively measured for impairment, related allowance				9,124	8,773
Individually measured for impairment, related allowance				0	0
Total measured for impairment, related allowance	8,773	4,164	4,233	9,124	8,773
Specific Allowance [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	217,959
Ending balance	208,465	217,959
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment				16,965,550	16,758,676
Individually measured for impairment, recorded investment				187,874	230,984
Total				17,153,424	16,989,660
Related allowance [Abstract]
Collectively measured for impairment, related allowance				199,634	217,116
Individually measured for impairment, related allowance				8,831	843
Total measured for impairment, related allowance	217,959	217,959		208,465	217,959
Nonspecific Allowance [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	28,200	30,126	28,045
Provision for loan losses	(5,983)	(1,926)	2,081
Loans charged off	0	0	0
Recoveries	0	0	0
Ending balance	22,217	28,200	30,126
Accrual for off-balance sheet credit losses [Roll Forward]
Beginning balance	0	0	0
Provision for off-balance sheet credit risk	0	0	0
Ending balance	0	0	0
Total provision for credit losses	(5,983)	(1,926)	2,081
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 measured for impairment, related allowance	$ 28,200	$ 30,126	$ 28,045	$ 22,217	$ 28,200