Loans, Loans by Credit Quality Indicator (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 20,784,991	$ 20,054,716
Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	20,171,876	19,251,353
Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	227,692	176,918
Potential Problem
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	176,877	351,142
Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	208,546	275,303
Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	6,399,693	6,489,014
Commercial and industrial | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	6,015,884	5,937,119
Commercial and industrial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	157,245	141,328
Commercial and industrial | Potential Problem
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	113,778	227,196
Commercial and industrial | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	112,786	183,371
Commercial real estate — owner occupied
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	802,209	897,724
Commercial real estate — owner occupied | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	723,291	805,871
Commercial real estate — owner occupied | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	14,181	17,785
Commercial real estate — owner occupied | Potential Problem
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	41,997	64,524
Commercial real estate — owner occupied | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	22,740	9,544
Commercial and business lending
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	7,201,902	7,386,738
Commercial and business lending | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	6,739,175	6,742,990
Commercial and business lending | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	171,426	159,113
Commercial and business lending | Potential Problem
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	155,775	291,720
Commercial and business lending | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	135,526	192,915
Commercial real estate — investor
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,315,254	3,574,732
Commercial real estate — investor | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,266,389	3,491,217
Commercial real estate — investor | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	24,845	14,236
Commercial real estate — investor | Potential Problem
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	19,291	51,228
Commercial real estate — investor | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	4,729	18,051
Real estate construction
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,451,684	1,432,497
Real estate construction | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,421,504	1,429,083
Real estate construction | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	29,206	105
Real estate construction | Potential Problem
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	0	2,465
Real estate construction | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	974	844
Commercial real estate lending
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	4,766,938	5,007,229
Commercial real estate lending | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	4,687,893	4,920,300
Commercial real estate lending | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	54,051	14,341
Commercial real estate lending | Potential Problem
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	19,291	53,693
Commercial real estate lending | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	5,703	18,895
Total commercial
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	11,968,840	12,393,967
Total commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	11,427,068	11,663,290
Total commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	225,477	173,454
Total commercial | Potential Problem
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	175,066	345,413
Total commercial | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	141,229	211,810
Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	7,546,534	6,332,327
Residential mortgage | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	7,490,860	6,275,162
Residential mortgage | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	426	1,314
Residential mortgage | Potential Problem
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,616	5,615
Residential mortgage | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	53,632	50,236
Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	883,804	934,443
Home equity | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	868,958	919,740
Home equity | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,137	1,588
Home equity | Potential Problem
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	195	114
Home equity | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	13,514	13,001
Other consumer
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	385,813	393,979
Other consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	384,990	393,161
Other consumer | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	652	562
Other consumer | Potential Problem
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	0	0
Other consumer | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	171	256
Total consumer
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	8,816,151	7,660,749
Total consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	8,744,808	7,588,063
Total consumer | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	2,215	3,464
Total consumer | Potential Problem
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,811	5,729
Total consumer | Nonaccrual
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 67,317	$ 63,493