Borrowings - Schedule of Long-term Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	May 06, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Stated Rate		5.50%
Subordinated debt obligations	$ 586,145		$ 428,470
Hedge related adjustment(5)	10,975		2,773
Discounted lease rentals	$ 75,770		$ 100,882
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtAndCapitalLeaseObligations		us-gaap:LongTermDebtAndCapitalLeaseObligations
Total long-term borrowings	$ 1,374,732		$ 2,354,448
Purchase accounting premiums	9,800		12,100
Purchase accounting discounts	(1,500)		(1,600)
FHLB advances due 2021 to 2025
Debt Instrument [Line Items]
FHLB advances	709,848		1,822,058
FHLB advances	150,000		110,000
Advances payable	$ 700,000		1,800,000
FHLB advances due 2021 to 2025 | Minimum
Debt Instrument [Line Items]
Stated Rate	0.39%
FHLB advances due 2021 to 2025 | Maximum
Debt Instrument [Line Items]
Stated Rate	2.72%
Subordinated debt obligation due 2022
Debt Instrument [Line Items]
Stated Rate	6.25%
Subordinated debt obligations	$ 109,597		109,338
Subordinated debt obligation due 2025
Debt Instrument [Line Items]
Stated Rate	4.60%
Subordinated debt obligations	$ 148,913		148,681
Subordinated debt obligation due 2029
Debt Instrument [Line Items]
Stated Rate	4.13%
Subordinated debt obligations	$ 148,933		148,657
Subordinated debt obligation due 2030
Debt Instrument [Line Items]
Stated Rate	5.50%
Subordinated debt obligations	$ 148,637		0
Subordinated debt securities due 2032-2035
Debt Instrument [Line Items]
Subordinated debt obligations	19,090		19,021
Amounts payable	$ 20,600		20,600
Subordinated debt securities due 2032-2035 | Minimum
Debt Instrument [Line Items]
Stated Rate	1.67%
Subordinated debt securities due 2032-2035 | Maximum
Debt Instrument [Line Items]
Stated Rate	3.50%
Discounted lease rentals due 2021 to 2025 | Minimum
Debt Instrument [Line Items]
Stated Rate	2.03%
Discounted lease rentals due 2021 to 2025 | Maximum
Debt Instrument [Line Items]
Stated Rate	6.07%
Finance lease obligation due 2038
Debt Instrument [Line Items]
Stated Rate	3.73%
Finance lease obligation	$ 2,969		$ 3,038
Subordinated bank notes | Three-Month LIBOR | Subordinated debt securities due 2032-2035 | Minimum
Debt Instrument [Line Items]
Floating-rate	1.45%
Subordinated bank notes | Three-Month LIBOR | Subordinated debt securities due 2032-2035 | Maximum
Debt Instrument [Line Items]
Floating-rate	3.25%