Debt - Additional Information (Detail) In Millions, unless otherwise specified	12 Months Ended								12 Months Ended	1 Months Ended	12 Months Ended					12 Months Ended
	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2012 Federal Home Loan Bank Borrowings USD ($)	Dec. 31, 2009 Federal Home Loan Bank Borrowings USD ($) Y	Dec. 31, 2010 Federal Home Loan Bank Borrowings USD ($)	Dec. 31, 2012 Collateralized Borrowing Program USD ($)	Dec. 31, 2011 Collateralized Borrowing Program USD ($)	Jan. 31, 2013 Repayment of Debt [Member] Federal Home Loan Bank Borrowings USD ($)	Jan. 31, 2013 Repayment of Debt [Member] Collateralized Borrowing Program USD ($)	Dec. 31, 2012 Senior Unsecured Notes Issued On June 17, 2011 USD ($)	Dec. 31, 2012 Senior unsecured Notes Issued On February 23, 2010 USD ($)	Dec. 31, 2012 Senior Unsecured Notes Issued On October 16, 1995 USD ($)	Dec. 31, 2011 Senior Unsecured Notes Issued On October 16, 1995 USD ($)	Dec. 31, 2009 Senior Unsecured Notes Issued On October 16, 1995 USD ($)	Dec. 31, 2012 Series B 8.207% Subordinated Deferrable Interest Debentures USD ($)	Dec. 31, 2011 Series B 8.207% Subordinated Deferrable Interest Debentures USD ($)	Dec. 31, 2012 AIX Holdings, Inc. USD ($)	Dec. 31, 2011 AIX Holdings, Inc. USD ($)	Dec. 31, 2011 Professionals Direct, Inc. USD ($)	Dec. 31, 2012 Maturing November 16, 2034 EUR (€)	Dec. 31, 2012 Floating Rate Subordinated Unsecured Notes Maturing September 21, 2036 USD ($)	Dec. 31, 2012 Unsecured Debt USD ($)
Debt Instrument [Line Items]
Aggregate principal amount											$ 300.0	$ 200.0	$ 200.0	$ 121.6	$ 120.9	$ 59.7	$ 59.7				€ 12.0	$ 50.0
Debt instrument interest rate					5.50%						6.375%	7.50%	7.625%
Issuance date												Feb. 23, 2010
Maturity date				Jul. 30, 2020							Jun. 15, 2025	Mar. 01, 2020	Oct. 15, 2025			Feb. 03, 2027					Nov. 16, 2034	Sep. 21, 2036
Interest rate of Series B Subordinated Deferrable Interest Debentures																8.207%
Repurchase of subordinated debt																	69.5
Repurchase of subordinated debt at cost																	72.1
Loss on the repurchase of debt	(5.1)	(2.3)	(2.0)														2.6						5.1
Repurchase of capital securities																		7.0	2.6	3.0
Debt instrument interest rate, agreed margin																					3.75%	3.10%
Debt instrument, carrying value	851.9	917.8																					60.3
Advance received					125.0
Length of debt instrument					20
Securities borrowed						46.3
Weighted average fixed interest rate				3.88%
Repayment of FHLBB community development advances									46.3	46.3
Interest costs, capitalized during period				46.3
Securities held as collateral, at fair value							200.8	205.7
Aggregate amount of borrowings							171.3	163.9
Interest expense	$ 61.9	$ 55.0	$ 44.3