Fair Values Measurements - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held-for-sale	$ 114,831	$ 18,070
Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	5,579,835	2,470,065
Loans held-for-sale	114,831	18,070
Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	0	0
Loans held-for-sale	0	0
Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	5,579,407	2,470,061
Loans held-for-sale	114,831	0
Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	428	4
Loans held-for-sale	0	18,070
Carrying Amount | Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held-for-sale	100,216	72,594
Net loans and leases	30,916,379	16,384,702
Loan servicing rights	55,301
Securitization receivable	19,624	19,432
Deferred fees on commitments to extend credit	18,485	18,555
Total financial instrument assets	31,544,243	16,735,789
Certificates of deposits	8,385,972	4,790,680
Long-term borrowings	860,482	1,449,472
Deferred fees on standby letters of credit	38	77
Total financial instrument liabilities	9,246,492	6,240,229
Carrying Amount | Recurring | FHLB and FRB stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	290,238	91,654
Carrying Amount | Recurring | Investment securities held-to-maturity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	144,000	148,852
Estimated Fair Value | Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held-for-sale	103,290	74,078
Net loans and leases	30,897,046	16,399,551
Loan servicing rights	55,301
Securitization receivable	19,355	19,025
Deferred fees on commitments to extend credit	18,485	18,555
Total financial instrument assets	31,533,643	16,752,130
Certificates of deposits	8,385,496	4,820,442
Long-term borrowings	875,864	1,451,550
Deferred fees on standby letters of credit	38	77
Total financial instrument liabilities	9,261,398	6,272,069
Estimated Fair Value | Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held-for-sale	0	0
Net loans and leases	0	0
Loan servicing rights	0
Securitization receivable	0	0
Deferred fees on commitments to extend credit	0	0
Total financial instrument assets	0	0
Certificates of deposits	0	0
Long-term borrowings	0	0
Deferred fees on standby letters of credit	0	0
Total financial instrument liabilities	0	0
Estimated Fair Value | Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held-for-sale	84,959	0
Net loans and leases	0	0
Loan servicing rights	0
Securitization receivable	0	0
Deferred fees on commitments to extend credit	18,485	18,555
Total financial instrument assets	539,934	256,676
Certificates of deposits	8,385,496	4,820,442
Long-term borrowings	875,864	1,451,550
Deferred fees on standby letters of credit	38	77
Total financial instrument liabilities	9,261,398	6,272,069
Estimated Fair Value | Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held-for-sale	18,331	74,078
Net loans and leases	30,897,046	16,399,551
Loan servicing rights	55,301
Securitization receivable	19,355	19,025
Deferred fees on commitments to extend credit	0	0
Total financial instrument assets	30,993,709	16,495,454
Certificates of deposits	0	0
Long-term borrowings	0	0
Deferred fees on standby letters of credit	0	0
Total financial instrument liabilities	0	0
Estimated Fair Value | Recurring | FHLB and FRB stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	290,238	91,654
Estimated Fair Value | Recurring | FHLB and FRB stocks | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	0	0
Estimated Fair Value | Recurring | FHLB and FRB stocks | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	290,238	91,654
Estimated Fair Value | Recurring | FHLB and FRB stocks | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	0	0
Estimated Fair Value | Recurring | Investment securities held-to-maturity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	149,928	149,267
Estimated Fair Value | Recurring | Investment securities held-to-maturity | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	0	0
Estimated Fair Value | Recurring | Investment securities held-to-maturity | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	146,252	146,467
Estimated Fair Value | Recurring | Investment securities held-to-maturity | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities carried at fair value	$ 3,676	$ 2,800