Mortgage Servicing Rights - Changes in Amortized MSRs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Changes in amortized mortgage servicing rights
Balance at beginning of period	$ 18,659	$ 15,800	$ 13,196	$ 16,809
Originations	21	1	28	4
Purchases			6,444
Amortization	(1,020)	(924)	(2,008)	(1,936)
Balance at end of period	17,660	14,877	17,660	14,877
Fair value of amortized MSRs at end of period	$ 28,344	$ 23,263	28,344	23,263	$ 29,048	$ 21,697	$ 24,495	$ 25,160
Impairment of MSRs recorded			$ 0	$ 0