Investments in debt and equity securities available-for-sale - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Losses
Less than 12 months	$ 3,835	$ 4,801
More than 12 months	535	440
Total	4,370	5,241
Fair values
Less than 12 months	190,114	239,812
More than 12 months	11,630	5,410
Total	201,744	245,222
Municipal
Losses
Less than 12 months	55	575
More than 12 months	136	0
Total	191	575
Fair values
Less than 12 months	3,771	32,038
More than 12 months	4,353	0
Total	8,124	32,038
Corporate
Losses
Less than 12 months	206	2,189
More than 12 months	56	0
Total	262	2,189
Fair values
Less than 12 months	39,708	119,965
More than 12 months	1,638	0
Total	41,346	119,965
Foreign
Losses
Less than 12 months	3,191	1,427
More than 12 months	233	193
Total	3,424	1,620
Fair values
Less than 12 months	131,992	70,012
More than 12 months	5,071	3,160
Total	137,063	73,172
U.S. Treasury Bonds
Losses
Less than 12 months	128	186
More than 12 months	0	0
Total	128	186
Fair values
Less than 12 months	8,293	11,847
More than 12 months	0	0
Total	8,293	11,847
Equity securities
Losses
Less than 12 months	255	424
More than 12 months	110	247
Total	365	671
Fair values
Less than 12 months	6,350	5,950
More than 12 months	568	2,250
Total	$ 6,918	$ 8,200