Loans - Past Due, Non-Accrual and Current Non-Covered Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	$ 84,454	$ 47,225
Total Past Due Loans	117,746	127,706
Current Non-PCI Loans	18,469,269	16,816,659
Total Non-PCI Loans	18,587,015	16,944,365
30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	25,812	48,900
60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	4,371	28,491
Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	3,109	3,090
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	53,596	20,890
Total Past Due Loans	62,469	25,084
Current Non-PCI Loans	2,959,055	2,523,981
Total Non-PCI Loans	3,021,524	2,549,065
Commercial and industrial | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	6,780	3,650
Commercial and industrial | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,533	544
Commercial and industrial | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	560	0
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	8,552	12,060
Total Past Due Loans	13,077	55,104
Current Non-PCI Loans	10,093,270	9,316,711
Total Non-PCI Loans	10,106,347	9,371,815
Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	4,498	24,172
Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	18,845
Commercial real estate | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	27	27
Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	7,452	11,328
Total Past Due Loans	11,802	22,578
Current Non-PCI Loans	9,097,843	8,539,273
Total Non-PCI Loans	9,109,645	8,561,851
Commercial real estate | Commercial real estate | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	4,323	11,223
Commercial real estate | Commercial real estate | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	0
Commercial real estate | Commercial real estate | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	27	27
Commercial real estate | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	1,100	732
Total Past Due Loans	1,275	32,526
Current Non-PCI Loans	995,427	777,438
Total Non-PCI Loans	996,702	809,964
Commercial real estate | Construction | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	175	12,949
Commercial real estate | Construction | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	18,845
Commercial real estate | Construction | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	0
Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	19,303	12,405
Total Past Due Loans	31,566	35,756
Current Non-PCI Loans	3,019,863	2,681,988
Total Non-PCI Loans	3,051,429	2,717,744
Residential mortgage | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	7,961	12,669
Residential mortgage | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	1,978	7,903
Residential mortgage | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	2,324	2,779
Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	3,003	1,870
Total Past Due Loans	10,634	11,762
Current Non-PCI Loans	2,397,081	2,293,979
Total Non-PCI Loans	2,407,715	2,305,741
Consumer loans | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	6,573	8,409
Consumer loans | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	860	1,199
Consumer loans | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	198	284
Consumer loans | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	2,804	1,777
Total Past Due Loans	3,768	2,880
Current Non-PCI Loans	354,275	370,751
Total Non-PCI Loans	358,043	373,631
Consumer loans | Home equity | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	686	1,009
Consumer loans | Home equity | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	278	94
Consumer loans | Home equity | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	0	0
Consumer loans | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	80	73
Total Past Due Loans	6,271	7,038
Current Non-PCI Loans	1,275,032	1,201,766
Total Non-PCI Loans	1,281,303	1,208,804
Consumer loans | Automobile | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	5,482	5,707
Consumer loans | Automobile | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	545	987
Consumer loans | Automobile | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	164	271
Consumer loans | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual Loans	119	20
Total Past Due Loans	595	1,844
Current Non-PCI Loans	767,774	721,462
Total Non-PCI Loans	768,369	723,306
Consumer loans | Other consumer | 30-59 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	405	1,693
Consumer loans | Other consumer | 60-89 Days Past Due Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	37	118
Consumer loans | Other consumer | Accruing Loans 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due Loans	$ 34	$ 13