Investments - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended				3 Months Ended				3 Months Ended		3 Months Ended												3 Months Ended
	Mar. 31, 2014 Security	Dec. 31, 2013 Security	Mar. 31, 2014 Held-to-maturity Securities [Member]	Dec. 31, 2013 Held-to-maturity Securities [Member]	Mar. 31, 2013 Held-to-maturity Securities [Member]	Mar. 31, 2014 SeaBright Pavonia Companies, Arden and Atrium [Member]	Dec. 31, 2013 SeaBright Pavonia Companies, Arden and Atrium [Member]	Mar. 31, 2014 Private Equity Funds [Member]	Dec. 31, 2013 Private Equity Funds [Member]	Mar. 31, 2014 Real Estate Debt Fund [Member]	Dec. 31, 2013 Real Estate Debt Fund [Member]	Mar. 31, 2014 Fixed Income Hedge Funds [Member]	Dec. 31, 2013 Fixed Income Hedge Funds [Member]	Mar. 31, 2014 U.S. Government and Agency [Member]	Dec. 31, 2013 U.S. Government and Agency [Member]	Mar. 31, 2014 U.S. Government and Agency [Member] Held-to-maturity Securities [Member]	Dec. 31, 2013 U.S. Government and Agency [Member] Held-to-maturity Securities [Member]	Mar. 31, 2014 U.S. Government and Agency [Member] Residential and commercial mortgaged-backed securities [Member]	Dec. 31, 2013 U.S. Government and Agency [Member] Residential and commercial mortgaged-backed securities [Member]	Mar. 31, 2014 Residential Mortgage-Backed [Member]	Dec. 31, 2013 Residential Mortgage-Backed [Member]	Mar. 31, 2014 Commercial Mortgage-Backed [Member]	Dec. 31, 2013 Commercial Mortgage-Backed [Member]	Mar. 31, 2014 Significant Unobservable Inputs (Level 3) [Member] Security	Mar. 31, 2013 Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2013 Significant Unobservable Inputs (Level 3) [Member]	Mar. 31, 2014 Significant Unobservable Inputs (Level 3) [Member] Held-to-maturity Securities [Member]	Dec. 31, 2013 Significant Unobservable Inputs (Level 3) [Member] Held-to-maturity Securities [Member]	Mar. 31, 2014 Significant Unobservable Inputs (Level 3) [Member] U.S. Government and Agency [Member]	Dec. 31, 2013 Significant Unobservable Inputs (Level 3) [Member] U.S. Government and Agency [Member]	Mar. 31, 2014 Significant Unobservable Inputs (Level 3) [Member] U.S. Government and Agency [Member] Held-to-maturity Securities [Member]	Dec. 31, 2013 Significant Unobservable Inputs (Level 3) [Member] U.S. Government and Agency [Member] Held-to-maturity Securities [Member]	Mar. 31, 2014 Significant Unobservable Inputs (Level 3) [Member] Residential Mortgage-Backed [Member]	Dec. 31, 2013 Significant Unobservable Inputs (Level 3) [Member] Residential Mortgage-Backed [Member]	Mar. 31, 2014 Significant Unobservable Inputs (Level 3) [Member] Commercial Mortgage-Backed [Member]	Dec. 31, 2013 Significant Unobservable Inputs (Level 3) [Member] Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities														$ 188,300,000	$ 177,900,000			$ 1,100,000	$ 12,500,000
Number of securities classified as available-for-sale in an unrealized loss position	97	135
Fair value of securities classified as available-for-sale in an unrealized loss position	89,533,000	121,830,000												7,234,000	11,416,000					1,516,000	13,632,000
Number of securities classified as available-for-sale in an unrealized loss position for twelve months or longer
Securities considered as other than temporarily impaired	0
Other-than-temporary impairment credit losses recognized	0
Other-than-temporary impairment credit losses	0
Other investments recorded in private equities	646,765,000	569,293,000						191,452,000	161,229,000	32,469,000	32,113,000
Percent of other investments recorded in private equities								2.80%	2.50%
Imposed lock-up period												3 years
Number of days to give notice for quarterly hedge fund redemptions												90 days
Investment in the funds will be eligible for redemption												Mar. 31, 2014
Redemption request description										A redemption request for this fund can be made 10 days after the date of any monthly valuation
Side-pockets investments	2,649,000	3,150,000										2,649,000	3,150,000
Number of corporate security																								1
Investments	4,665,922,000	4,660,411,000	827,391,000	799,559,000	0									415,811,000	468,289,000	18,622,000	18,132,000			237,896,000	235,964,000	127,664,000	114,637,000	302,008,000		270,903,000
Transfers between Levels 1 and 2	0	0
Fair value of changes in assets																								10,800,000	12,900,000
Increase of restricted assets						$ 607,200,000	$ 397,700,000