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, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	$ 737	$ 2,518
Fair values, Less than 12 months	40,084	126,438
Losses, More than 12 months	326	1,196
Fair values, More than 12 months	3,868	44,448
Total Losses	1,063	3,714
Total Fair Values	43,952	170,886
Municipal
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	49	9
Fair values, Less than 12 months	5,544	2,230
Losses, More than 12 months	0	3
Fair values, More than 12 months	0	1,615
Total Losses	49	12
Total Fair Values	5,544	3,845
Corporate
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	82	1,461
Fair values, Less than 12 months	12,415	83,565
Losses, More than 12 months	0	648
Fair values, More than 12 months	0	32,871
Total Losses	82	2,109
Total Fair Values	12,415	116,436
Foreign
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	28	322
Fair values, Less than 12 months	15,045	35,008
Losses, More than 12 months	13	160
Fair values, More than 12 months	1,574	3,155
Total Losses	41	482
Total Fair Values	16,619	38,163
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months		6
Fair values, Less than 12 months		1,195
Losses, More than 12 months		19
Fair values, More than 12 months		3,583
Total Losses		25
Total Fair Values		4,778
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	578	720
Fair values, Less than 12 months	7,080	4,440
Losses, More than 12 months	313	366
Fair values, More than 12 months	2,294	3,224
Total Losses	891	1,086
Total Fair Values	$ 9,374	$ 7,664