Investments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015 USD ($) Security fund	Dec. 31, 2014 USD ($) Security	Dec. 31, 2013 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of securities classified as available-for-sale in an unrealized loss position | Security	332	212
Number of securities classified as available-for-sale in an unrealized loss position for twelve months or longer | Security	124	120
Number of securities classified as held-to-maturity in an unrealized loss position | Security	109	61
Number of securities classified as held-to-maturity in an unrealized loss position for twelve months or longer | Security	53	57
Gates or side-pocket investments	$ 1,100,000
Unfunded commitments	140,200,000
Other investments, at cost	133,071,000	$ 0
Amount of net investment income included in earnings attributable to investments in life settlements	20,100,000
Impairment charges recognized in the period	0
Life insurance premiums payable in 2016	17,900,000
Life insurance premiums payable in 2017	17,900,000
Life insurance premiums payable in 2018	17,900,000
Life insurance premiums payable in 2019	18,100,000
Life insurance premiums payable in 2020	17,000,000
Proceeds from sales of available-for-sale securities	95,100,000	90,700,000	$ 20,500,000
Restricted assets, including restricted cash	$ 511,300,000
Fixed income hedge funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Imposed lock-up period (up to) (years)	3 years
Number of days to give notice for quarterly hedge fund redemptions	90 days
Real estate debt fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Redemption request description	A redemption request for this fund can be made 10 days after the date of any monthly valuation
Redemption request period (days)	10 days
CLO equity funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of CLO equity funds | fund	2
CLO Equity Fund One
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments, fair value	$ 4,400,000
CLO Equity Fund Two
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments, fair value	9,500,000
Private equities and private equity funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unfunded commitments	140,200,000
U.S. Government and Agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial mortgage-backed securities	359,400,000	263,400,000
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior secured loans	$ 94,400,000	$ 33,500,000
Minimum | CLO Equity Fund One
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
CLO equity fund pay out period (years)	2 years
Maximum | CLO Equity Fund One
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
CLO equity fund pay out period (years)	6 years