Investments in debt and equity securities available-for-sale - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Losses
Less than 12 months	$ 3,497	$ 4,801
More than 12 months	248	440
Total	3,745	5,241
Fair values
Less than 12 months	203,852	239,812
More than 12 months	3,023	5,410
Total	206,875	245,222
Municipal
Losses
Less than 12 months	429	575
More than 12 months	0	0
Total	429	575
Fair values
Less than 12 months	12,503	32,038
More than 12 months	0	0
Total	12,503	32,038
Corporate
Losses
Less than 12 months	1,216	2,189
More than 12 months	0	0
Total	1,216	2,189
Fair values
Less than 12 months	98,998	119,965
More than 12 months	0	0
Total	98,998	119,965
Foreign
Losses
Less than 12 months	1,095	1,427
More than 12 months	22	193
Total	1,117	1,620
Fair values
Less than 12 months	72,071	70,012
More than 12 months	1,550	3,160
Total	73,621	73,172
U.S. Treasury Bonds
Losses
Less than 12 months	165	186
More than 12 months	0	0
Total	165	186
Fair values
Less than 12 months	11,867	11,847
More than 12 months	0	0
Total	11,867	11,847
Equity securities
Losses
Less than 12 months	592	424
More than 12 months	226	247
Total	818	671
Fair values
Less than 12 months	8,413	5,950
More than 12 months	1,473	2,250
Total	$ 9,886	$ 8,200