Investments in debt and equity securities available-for-sale - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Losses
Less than 12 months	$ 3,486	$ 4,801
More than 12 months	29	440
Total	3,515	5,241
Fair values
Less than 12 months	184,422	239,812
More than 12 months	295	5,410
Total	184,717	245,222
Municipal
Losses
Less than 12 months	248	575
More than 12 months	0	0
Total	248	575
Fair values
Less than 12 months	10,021	32,038
More than 12 months	0	0
Total	10,021	32,038
Corporate
Losses
Less than 12 months	414	2,189
More than 12 months	0	0
Total	414	2,189
Fair values
Less than 12 months	56,365	119,965
More than 12 months	0	0
Total	56,365	119,965
Foreign
Losses
Less than 12 months	1,657	1,427
More than 12 months	15	193
Total	1,672	1,620
Fair values
Less than 12 months	95,724	70,012
More than 12 months	247	3,160
Total	95,971	73,172
U.S. Treasury Bonds
Losses
Less than 12 months	129	186
More than 12 months	0	0
Total	129	186
Fair values
Less than 12 months	10,824	11,847
More than 12 months	0	0
Total	10,824	11,847
Equity securities
Losses
Less than 12 months	1,038	424
More than 12 months	14	247
Total	1,052	671
Fair values
Less than 12 months	11,488	5,950
More than 12 months	48	2,250
Total	$ 11,536	$ 8,200