Investments in debt and equity securities available-for-sale - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	$ 514	$ 4,062
Fair Values, Less than 12 months	65,793	162,540
Losses, More than 12 months	1,397	2,192
Fair Values, More than 12 months	71,022	76,145
Total Losses	1,911	6,254
Total Fair Values	136,815	238,685
Municipal
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	2	770
Fair Values, Less than 12 months	494	27,686
Losses, More than 12 months	118	43
Fair Values, More than 12 months	10,747	628
Total Losses	120	813
Total Fair Values	11,241	28,314
Corporate
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	123	1,682
Fair Values, Less than 12 months	21,044	66,776
Losses, More than 12 months	928	1,546
Fair Values, More than 12 months	34,704	21,710
Total Losses	1,051	3,228
Total Fair Values	55,748	88,486
Foreign
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	24	1,539
Fair Values, Less than 12 months	42,924	63,039
Losses, More than 12 months	312	603
Fair Values, More than 12 months	22,016	53,807
Total Losses	336	2,142
Total Fair Values	64,940	116,846
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	1	60
Fair Values, Less than 12 months	241	3,772
Losses, More than 12 months	39	0
Fair Values, More than 12 months	3,555	0
Total Losses	40	60
Total Fair Values	3,796	3,772
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	364	11
Fair Values, Less than 12 months	1,090	1,267
Losses, More than 12 months	0	0
Fair Values, More than 12 months	0	0
Total Losses	364	11
Total Fair Values	$ 1,090	$ 1,267