Investments - Additional Information (Details) $ in Thousands	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) security	Jun. 30, 2018 USD ($) security	Sep. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Investment Holdings [Line Items]
Available-for-sale investments, cost	$ 454,603		$ 454,603	$ 182,325
Par value of mortgage-backed securities pledged to FRB	214,000		214,000	147,000
Gain on equity securities investment	8,000
Non-marketable securities investment	26,000		26,000	8,000
Low income housing tax credit investments	59,000		59,000	48,000
Low income housing tax credit investments, liability for unfunded commitments	33,000		33,000	37,000
Low income housing tax credit investments, tax credits and benefits			$ 800	900
Low income housing tax credit investments, expected tax benefits recognized			25.00%
Mortgage-backed securities
Investment Holdings [Line Items]
Available-for-sale investments, cost	$ 219,531		$ 219,531	$ 159,937
Number of securities in net loss position | security	35		35	74
Number of mortgage-backed securities | security	103		103	86
Number of mortgage backed securities sold | security		9
Proceeds from sale of securities		$ 41,000
Recognized loss from sale of securities		$ 2,000
Utah Housing Corporation bonds
Investment Holdings [Line Items]
Available-for-sale investments, cost	$ 19,474		$ 19,474	$ 22,388
Number of securities in net loss position | security	1		1
Number of mortgage-backed securities | security	3		3
U.S. government-sponsored enterprises
Investment Holdings [Line Items]
Available-for-sale investments, cost	$ 215,598		$ 215,598
Number of securities in net loss position | security	2		2
Number of government sponsored securities | security	11		11
Ginnie Mae | Mortgage-backed securities
Investment Holdings [Line Items]
Available-for-sale investments, cost	$ 56,000		$ 56,000
Number of securities in net loss position | security	16		16	34
Number of mortgage-backed securities | security	35		35	74
Fannie Mae | Mortgage-backed securities
Investment Holdings [Line Items]
Available-for-sale investments, cost	$ 103,000		$ 103,000
Freddie Mac | Mortgage-backed securities
Investment Holdings [Line Items]
Available-for-sale investments, cost	$ 60,000		$ 60,000