Debt Securities Available for Sale - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) security	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) security	Sep. 30, 2019 USD ($) security	Dec. 31, 2019 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 1,204,001,000		$ 1,204,001,000		$ 1,086,143,000
Debt securities available for sale, at fair value	1,245,300,000		1,245,300,000		1,098,336,000
Proceeds from sales of debt securities available for sale	0	$ 16,100,000	20,761,000	$ 31,811,000
Gross gains on sale of available-for-sale securities		1,200,000	369,000	1,500,000
Gross losses on sale of available-for-sale securities		0	0	0
Proceeds from calls of debt securities available for sale		0	6,600,000	$ 0
Sale and maturity of debt securities, available-for-sale	797,000		5,800,000
Calls of debt securities, available-for-sale, realized gain			1,000
Calls of debt securities, available-for-sale, unrealized loss			0
Number of debt securities called | security				1
Debt securities, available-for-sale, restricted	$ 800,900,000		$ 800,900,000		$ 462,000,000.0
Number of securities in unrealized loss position | security	41		41		97
Other-than-temporary impairments charge on debt securities available for sale	$ 0	$ 0	$ 0	$ 0
Mortgage-backed securities and collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,095,120,000		1,095,120,000		$ 968,165,000
Debt securities available for sale, at fair value	$ 1,134,106,000		$ 1,134,106,000		$ 979,881,000