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, 2018	Dec. 31, 2017
Losses
Less than 12 months	$ 7,086	$ 2,025
More than 12 months	2,513	2,138
Total	9,599	4,163
Fair values
Less than 12 months	395,832	220,615
More than 12 months	55,805	56,513
Total	451,637	277,128
Municipal
Losses
Less than 12 months	234	58
More than 12 months	270	117
Total	504	175
Fair values
Less than 12 months	25,959	17,023
More than 12 months	5,612	5,784
Total	31,571	22,807
Corporate
Losses
Less than 12 months	4,670	386
More than 12 months	192	111
Total	4,862	497
Fair values
Less than 12 months	241,586	81,632
More than 12 months	4,824	4,926
Total	246,410	86,558
Foreign
Losses
Less than 12 months	2,052	1,528
More than 12 months	1,804	1,727
Total	3,856	3,255
Fair values
Less than 12 months	122,531	116,130
More than 12 months	38,663	39,031
Total	161,194	155,161
U.S. Treasury Bonds
Losses
Less than 12 months	130	53
More than 12 months	247	183
Total	377	236
Fair values
Less than 12 months	5,756	5,830
More than 12 months	6,706	6,772
Total	$ 12,462	$ 12,602