Other Borrowings and Subordinated Notes and Debentures - Debt Components (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended
	Mar. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total other borrowings and subordinated debt		$ 1,865,148	$ 1,520,589
U.S. Bank National Association | Revolving credit facility
Debt Instrument [Line Items]
Total other borrowings and subordinated debt		$ 0	75,000
U.S. Bank National Association | Revolving credit facility | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)		1.50%
Total other borrowings
Debt Instrument [Line Items]
Total other borrowings and subordinated debt		$ 1,451,811	1,380,024
FHLB advances, net of discount, due 2018 to 2033, 1.33% to 7.37% secured by residential real estate loans
Debt Instrument [Line Items]
Total other borrowings and subordinated debt		$ 1,451,811	1,261,642
FHLB advances, net of discount, due 2018 to 2033, 1.33% to 7.37% secured by residential real estate loans | Minimum
Debt Instrument [Line Items]
Fixed rate (as percent)		1.33%
FHLB advances, net of discount, due 2018 to 2033, 1.33% to 7.37% secured by residential real estate loans | Maximum
Debt Instrument [Line Items]
Fixed rate (as percent)		7.37%
Notes payable, due 10/15/2020, 3.85%, fixed rate, unsecured
Debt Instrument [Line Items]
Total other borrowings and subordinated debt		$ 0	43,382
Fixed rate (as percent)		3.85%
Subordinated Notes and Debentures
Debt Instrument [Line Items]
Unamortized debt issuance costs		$ (3,554)	0
Total other borrowings and subordinated debt		413,337	140,565
Subordinated notes payable, due 4/1/2028, fixed-to-floating rate (fixed rate of 5.00% through 3/31/2023, floating rate of 2.15% above the three month LIBOR rate, reset quarterly)
Debt Instrument [Line Items]
Long-term debt, gross		$ 330,000	0
Fixed rate (as percent)	5.00%	5.00%
Subordinated notes payable, due 4/1/2028, fixed-to-floating rate (fixed rate of 5.00% through 3/31/2023, floating rate of 2.15% above the three month LIBOR rate, reset quarterly) | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)	2.15%
Debentures | Trust preferred securities, due 12/30/2033, floating rate of 2.80% above the three month LIBOR rate, reset quarterly, callable without penalty
Debt Instrument [Line Items]
Long-term debt, gross		$ 0	20,620
Debentures | Trust preferred securities, due 12/30/2033, floating rate of 2.80% above the three month LIBOR rate, reset quarterly, callable without penalty | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)		2.80%
Debentures | Trust preferred securities, net of discount, due 6/30/2035, floating rate of 1.75% above the three month LIBOR rate, reset quarterly, callable without penalty
Debt Instrument [Line Items]
Long-term debt, gross		$ 9,344	9,327
Debentures | Trust preferred securities, net of discount, due 6/30/2035, floating rate of 1.75% above the three month LIBOR rate, reset quarterly, callable without penalty | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)		1.75%
Debentures | Trust preferred securities, net of discount, due 9/15/2037, floating rate of 1.37% above the three month LIBOR rate, reset quarterly
Debt Instrument [Line Items]
Long-term debt, gross		$ 10,310	10,284
Debentures | Trust preferred securities, net of discount, due 9/15/2037, floating rate of 1.37% above the three month LIBOR rate, reset quarterly | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)		1.37%
Debentures | Trust preferred securities, net of discount, due 12/5/2033, floating rate of 2.88% above the three month LIBOR rate, reset quarterly, callable without penalty
Debt Instrument [Line Items]
Long-term debt, gross		$ 0	5,156
Debentures | Trust preferred securities, net of discount, due 12/5/2033, floating rate of 2.88% above the three month LIBOR rate, reset quarterly, callable without penalty | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)		2.88%
Debentures | Trust preferred securities, net of discount, due 10/18/2034, floating rate of 2.00% above the three month LIBOR rate, reset quarterly, callable without penalty
Debt Instrument [Line Items]
Long-term debt, gross		$ 5,155	5,148
Debentures | Trust preferred securities, net of discount, due 10/18/2034, floating rate of 2.00% above the three month LIBOR rate, reset quarterly, callable without penalty | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)		2.00%
Debentures | Trust preferred securities, net of discount, due 6/6/2037, floating rate of 1.57% above the three month LIBOR rate, reset quarterly, callable without penalty
Debt Instrument [Line Items]
Long-term debt, gross		$ 10,310	10,288
Debentures | Trust preferred securities, net of discount, due 6/6/2037, floating rate of 1.57% above the three month LIBOR rate, reset quarterly, callable without penalty | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)		1.57%
Debentures | Trust preferred securities, due 12/15/2035, floating rate of 1.45% above the three month LIBOR rate, reset quarterly, callable without penalty
Debt Instrument [Line Items]
Long-term debt, gross		$ 6,702	6,702
Debentures | Trust preferred securities, due 12/15/2035, floating rate of 1.45% above the three month LIBOR rate, reset quarterly, callable without penalty | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)		1.45%
Debentures | Trust preferred securities, due 6/26/2033, floating rate of 3.10% above the three month LIBOR rate, reset quarterly, callable without penalty
Debt Instrument [Line Items]
Long-term debt, gross		$ 0	20,619
Debentures | Trust preferred securities, due 6/26/2033, floating rate of 3.10% above the three month LIBOR rate, reset quarterly, callable without penalty | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)		3.10%
Debentures | Trust preferred securities, due 10/7/2033, floating rate of 2.85% above the three month LIBOR rate, reset quarterly, callable without penalty
Debt Instrument [Line Items]
Long-term debt, gross		$ 25,774	25,774
Debentures | Trust preferred securities, due 10/7/2033, floating rate of 2.85% above the three month LIBOR rate, reset quarterly, callable without penalty | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)		2.85%
Debentures | Trust preferred securities, due 9/15/2037, floating rate of 2.00% above the three month LIBOR rate, reset quarterly, callable without penalty
Debt Instrument [Line Items]
Long-term debt, gross		$ 0	8,248
Debentures | Trust preferred securities, due 9/15/2037, floating rate of 2.00% above the three month LIBOR rate, reset quarterly, callable without penalty | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)		2.00%
Debentures | Other subordinated debentures, net of discount, due 9/30/2023, floating rate equal to daily average of prime rate, reset quarterly
Debt Instrument [Line Items]
Long-term debt, gross		$ 19,296	$ 18,399