Loans and Allowance for Credit Losses for Loans - Past Due, Non-Accrual and Current Non-Covered Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	$ 205,930	$ 93,058
Total Past Due Loans	365,318	161,284
Current Loans	30,062,749	22,908,325
Total Loans	30,428,067
Non-Accrual Loans Without Allowance for Credit Losses	67,818
PCI Loans		6,629,599
30-59 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	117,036	46,593
60-89 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	35,252	14,315
90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	7,100	7,318
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	132,622	68,636
Total Past Due Loans	154,075	86,549
Current Loans	4,844,656	4,057,434
Total Loans	4,998,731
Non-Accrual Loans Without Allowance for Credit Losses	17,149
PCI Loans		682,014
Commercial and industrial | 30-59 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	9,780	11,700
Commercial and industrial | 60-89 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	7,624	2,227
Commercial and industrial | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	4,049	3,986
Commercial real estate
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	44,588	9,360
Total Past Due Loans	109,544	19,354
Current Loans	18,007,738	12,379,256
Total Loans	18,117,282
Non-Accrual Loans Without Allowance for Credit Losses	40,704
PCI Loans		5,245,149
Commercial real estate | 30-59 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	48,783	4,046
Commercial real estate | 60-89 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	16,012	5,369
Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	161	579
Commercial real estate | Commercial real estate
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	41,616	9,004
Total Past Due Loans	99,404	16,169
Current Loans	16,290,832	10,886,724
Total Loans	16,390,236
Non-Accrual Loans Without Allowance for Credit Losses	37,865
PCI Loans		5,093,848
Commercial real estate | Commercial real estate | 30-59 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	41,664	2,560
Commercial real estate | Commercial real estate | 60-89 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	15,963	4,026
Commercial real estate | Commercial real estate | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	161	579
Commercial real estate | Construction
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	2,972	356
Total Past Due Loans	10,140	3,185
Current Loans	1,716,906	1,492,532
Total Loans	1,727,046
Non-Accrual Loans Without Allowance for Credit Losses	2,839
PCI Loans		151,301
Commercial real estate | Construction | 30-59 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	7,119	1,486
Commercial real estate | Construction | 60-89 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	49	1,343
Commercial real estate | Construction | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Residential mortgage
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	24,625	12,858
Total Past Due Loans	74,695	36,235
Current Loans	4,404,287	3,760,707
Total Loans	4,478,982
Non-Accrual Loans Without Allowance for Credit Losses	9,834
PCI Loans		580,169
Residential mortgage | 30-59 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	38,965	17,143
Residential mortgage | 60-89 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	9,307	4,192
Residential mortgage | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,798	2,042
Consumer loans
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	4,095	2,204
Total Past Due Loans	27,004	19,146
Current Loans	2,806,068	2,710,928
Total Loans	2,833,072
Non-Accrual Loans Without Allowance for Credit Losses	131
PCI Loans		122,267
Consumer loans | 30-59 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	19,508	13,704
Consumer loans | 60-89 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	2,309	2,527
Consumer loans | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,092	711
Consumer loans | Home equity
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	3,845	1,646
Total Past Due Loans	6,979	2,777
Current Loans	474,772	373,243
Total Loans	481,751
Non-Accrual Loans Without Allowance for Credit Losses	131
PCI Loans		111,252
Consumer loans | Home equity | 30-59 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	2,625	1,051
Consumer loans | Home equity | 60-89 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	509	80
Consumer loans | Home equity | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	0	0
Consumer loans | Automobile
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	250	334
Total Past Due Loans	16,079	14,078
Current Loans	1,420,655	1,437,274
Total Loans	1,436,734
Non-Accrual Loans Without Allowance for Credit Losses	0
PCI Loans		271
Consumer loans | Automobile | 30-59 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	13,447	11,482
Consumer loans | Automobile | 60-89 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	1,756	1,581
Consumer loans | Automobile | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	626	681
Consumer loans | Other consumer
Financing Receivable, Past Due [Line Items]
Non-Accrual Loans	0	224
Total Past Due Loans	3,946	2,291
Current Loans	910,641	900,411
Total Loans	914,587
Non-Accrual Loans Without Allowance for Credit Losses	0
PCI Loans		10,744
Consumer loans | Other consumer | 30-59 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	3,436	1,171
Consumer loans | Other consumer | 60-89 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	44	866
Consumer loans | Other consumer | 90 Days or More Past Due Loans
Financing Receivable, Past Due [Line Items]
Past Due Loans	$ 466	$ 30