Debt Securities Available for Sale - Narrative (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, amortized cost		$ 1,086,143,000	$ 1,049,932,000
Debt securities available for sale, at fair value		1,098,336,000	1,032,868,000
Proceeds from sales of debt securities available for sale	$ 0	65,198,000	11,513,000	$ 0	$ 187,376,000
Debt securities, available-for-sale securities, gross realized gains		2,200,000			1,500,000
Debt securities, available-for-sale, gross unrealized losses		22,000			3,200,000
Proceeds from calls and maturities of debt securities, available-for-sale		24,100,000	11,500,000		17,200,000
Calls of debt securities, available-for-sale, realized gain		174,000	116,000
Calls of debt securities, available-for-sale, unrealized loss		$ 0	0
Debt securities, available-for-sale, number of matured debt securities | security		1
Proceeds from sale and maturity of debt securities, available for sale		$ 797,000	2,400,000
Debt securities, available-for-sale, restricted		$ 462,000,000.0	$ 232,700,000
Number of unrealized loss positions | security		97	151
Other-than-temporary impairment loss, debt securities, available for sale	$ 0	$ 0	$ 0		$ 0
Mortgage-backed securities and collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, amortized cost		968,165,000	934,631,000
Debt securities available for sale, at fair value		$ 979,881,000	$ 920,007,000