Significant New Business - Additional Information (Detail) £ in Millions	Feb. 17, 2016 USD ($)	Nov. 30, 2015 USD ($)	Sep. 30, 2015 USD ($) Agreement	May. 27, 2015 USD ($) trust Agreement	Jan. 15, 2015 USD ($)	Jan. 01, 2013 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jan. 01, 2014 USD ($)	Jan. 01, 2014 GBP (£)	Apr. 26, 2013 USD ($)	Jan. 01, 2013 GBP (£)
Business Acquisition [Line Items]
Funds held by reinsured companies							$ 109,358,000	$ 134,628,000
Net insurance reserve assumed									$ 28,100,000	£ 17.0
Percentage of underwriting capacity provided for Syndicate						100.00%
Gross insurance reserves						$ 51,400,000						£ 33.8
Sun Life
Business Acquisition [Line Items]
Percentage of reinsurance agreements entered			100.00%
Reinsurance reserve assumed			$ 128,300,000
Approximate value of additional funds transferred to trust			30,600,000
Limited parental guarantee obligations, initial amount (up to)			$ 36,800,000
Number of agreements entered | Agreement			2
Reinsurance reserves received			$ 122,500,000
Deferred reinsurance receivable included in other assets			$ 5,800,000
Voya Financial
Business Acquisition [Line Items]
Percentage of reinsurance agreements entered				100.00%
Reinsurance reserve assumed				$ 572,400,000
Approximate value of additional funds transferred to trust				$ 67,200,000
Limited parental guarantee obligations, initial amount (up to)							$ 58,000,000
Number of agreements entered | Agreement				2
Reinsurance reserves received				$ 307,000,000
Deferred reinsurance receivable included in other assets				$ 265,400,000
Number of reinsurance collateral trusts | trust				2
The Doctors Company and its affiliates
Business Acquisition [Line Items]
Purchase price		$ 29,500,000
Reciprocal of America | Reinsurance Agreement under 2015
Business Acquisition [Line Items]
Reinsurance agreement date					Jan. 15, 2015
Net insurance reserve assumed					$ 162,100,000
American Physicians
Business Acquisition [Line Items]
Total assets and liabilities assumed gross reinsurance reserves received											$ 35,300,000
Allianz | Subsequent Event
Business Acquisition [Line Items]
Percentage of reinsurance agreements entered	50.00%
Reinsurance reserve assumed	$ 1,100,000,000
Reinsurance premium assumed by counterparty	1,100,000,000
Approximate value of additional funds transferred to trust	$ 110,000,000
San Francisco Reinsurance Company | Subsequent Event
Business Acquisition [Line Items]
Percentage of reinsurance agreements entered	50.00%
Funds held by reinsured companies	$ 2,200,000,000
Limited Parental Guarantee | Allianz | Subsequent Event
Business Acquisition [Line Items]
Limited parental guarantee obligations, initial amount (up to)	$ 270,000,000.0