Other Borrowings and Subordinated Debentures - Long-term Debt (Details) - USD ($)	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total other borrowings and subordinated debt	$ 1,699,400,000		$ 1,520,589,000
Revolving Credit Facility | U.S. Bank National Association
Debt Instrument [Line Items]
Total other borrowings and subordinated debt	0	$ 0	75,000,000
Total other borrowings
Debt Instrument [Line Items]
Total other borrowings and subordinated debt	1,345,450,000		1,380,024,000
FHLB advances, net of discount, due 2019 to 2033, 1.38% to 7.37% secured by residential real estate loans
Debt Instrument [Line Items]
Total other borrowings and subordinated debt	1,345,450,000		1,261,642,000
Notes payable, due 10/15/2020, 3.85%, fixed rate, unsecured
Debt Instrument [Line Items]
Total other borrowings and subordinated debt	0		43,382,000
Total subordinated notes and debentures
Debt Instrument [Line Items]
Unamortized debt issuance costs	(3,372,000)		0
Total other borrowings and subordinated debt	353,950,000		140,565,000
Subordinated Debt | 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]
Total other borrowings and subordinated debt, gross	330,000,000		0
Subordinated Debt | 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]
Total other borrowings and subordinated debt, gross	0		20,620,000
Subordinated Debt | 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]
Total other borrowings and subordinated debt, gross	0		9,327,000
Subordinated Debt | 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]
Total other borrowings and subordinated debt, gross	10,310,000		10,284,000
Subordinated Debt | 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]
Total other borrowings and subordinated debt, gross	0		5,156,000
Subordinated Debt | 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]
Total other borrowings and subordinated debt, gross	0		5,148,000
Subordinated Debt | 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]
Total other borrowings and subordinated debt, gross	10,310,000		10,288,000
Subordinated Debt | 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]
Total other borrowings and subordinated debt, gross	6,702,000		6,702,000
Subordinated Debt | 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]
Total other borrowings and subordinated debt, gross	0		20,619,000
Subordinated Debt | 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]
Total other borrowings and subordinated debt, gross	0		25,774,000
Subordinated Debt | 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]
Total other borrowings and subordinated debt, gross	0		8,248,000
Unsecured Debt | 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]
Total other borrowings and subordinated debt, gross	$ 0		$ 18,399,000