Mortgage Servicing Rights - Changes in Amortized MSRs (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Changes in amortized mortgage servicing rights
Balance at beginning of period	$ 13,196,000	$ 16,809,000	$ 16,809,000
Originations	7,000	3,000
Purchases	6,444,000
Amortization	(988,000)	(1,012,000)
Balance at end of period	18,659,000	15,800,000	13,196,000
Fair value of amortized MSRs at end of period	29,048,000	$ 24,495,000	21,697,000	$ 25,160,000
Impairment of MSRs recorded	$ 0		$ 0