Debt Securities Held to Maturity - Narrative (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security	Sep. 30, 2017 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost		$ 285,756,000	$ 262,143,000
Securities held-to-maturity, fair value		289,505,000	254,841,000
Proceeds from sale of held-to-maturity securities	$ 0	0	0	$ 0
Proceeds from calls and maturities of debt securities held to maturity	0	33,400,000	5,400,000	769,000
Debt securities, held-to-maturity, realized gain		24,000	0	0
Debt securities, held-to-maturity, sold, realized loss		0
Securities available-for-sale sold under agreements		$ 236,000,000.0	$ 187,000,000.0
Number of unrealized loss positions | security		22	88
Other-than-temporary impairment loss, debt securities, held-to-maturity, before tax	0	$ 0	$ 0	0
Transfer of securities from available for sale securities	$ 0	0	0	103,680,000
Transfer of securities from available for sale securities, fair value				$ 103,300,000
Mortgage-backed securities and collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost		265,756,000	238,739,000
Securities held-to-maturity, fair value		$ 269,545,000	$ 231,600,000