Debt Securities Available for Sale - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($) security	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 1,095,422,000		$ 1,095,422,000		$ 1,049,932,000
Debt securities available for sale, at fair value	1,115,905,000		1,115,905,000		1,032,868,000
Proceeds from sales of debt securities available for sale	16,100,000		31,811,000	$ 0
Gross gains on sale of available-for-sale securities	1,200,000		1,500,000	116,000
Gross losses on sale of available-for-sale securities	0		0	0
Proceeds from calls of debt securities available for sale	$ 0	$ 1,500,000	$ 767,000	11,500,000
Number of matured debt securities | security	1		1
Number of debt securities called | security		1	0
Sale and maturity of debt securities, available-for-sale		$ 0		0
Debt securities, available-for-sale, restricted	$ 203,400,000		$ 203,400,000		$ 232,700,000
Number of securities in unrealized loss position | security	41		41		151
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	981,205,000		981,205,000		$ 934,631,000
Debt securities available for sale, at fair value	$ 1,001,443,000		$ 1,001,443,000		$ 920,007,000