Commitments and Contingencies - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended				12 Months Ended	9 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended			9 Months Ended					9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Equity Securities Exxon Mobil Corporation	Sep. 30, 2012 Equity Securities Energy Sector Business	Dec. 31, 2011 Equity Securities Energy Sector Business	Sep. 30, 2012 Equity Securities Energy Sector Business Exxon Mobil Corporation	Dec. 31, 2011 Equity Securities Energy Sector Business Exxon Mobil Corporation	Jul. 20, 2012 Reinsurance Segment	Sep. 30, 2012 Reinsurance Segment	Sep. 30, 2012 Reinsurance Segment	Mar. 06, 2012 Reinsurance Segment Gain contingency on dispute previously in arbitration	Sep. 30, 2012 Insurance Segment Capitol Transamerica Corporation and Platte River Insurance Company	Sep. 30, 2012 Claims Related to Events Before Acquisition	Dec. 31, 2011 Claims Related to Events Before Acquisition	Jul. 31, 2005 Claims Related to Events Before Acquisition	Sep. 30, 2012 Claims Related to Events Before Acquisition First $100 million	Sep. 30, 2012 Claims Related to Events Before Acquisition Second $100 million	Sep. 30, 2012 Litigation Related to Acquisition of Transatlantic Maximum
Commitments and Contingencies Disclosure [Line Items]
Legal settlement, petition to be filed for an award of attorneys' fees and expenses																				$ 500,000
Settlement payment awarded										75,000,000
Finite lived intangible assets													36,000,000
Pre-tax net benefit resulting from legal settlement											23,500,000	23,500,000
Indemnification obligation, maximum															150,000,000
Indemnification obligation, percentage																		100.00%	50.00%
Indemnification obligation, expiration date															Jul. 31, 2016
Reserve for products liability indemnification															100,000	200,000	600,000
Non-cancelable operating leases, expiration date			2022
Reserves for unpaid loss and loss adjustment expenses, gross	511,200,000		511,200,000								497,700,000	497,700,000		13,500,000
Reserves for unpaid loss and loss adjustment expenses, net	393,700,000		393,700,000								380,200,000	380,200,000		13,500,000
Concentration of market risk related to available-for-sale securities						572,800,000	573,300,000		254,300,000
Net realized capital gains	$ 12,388,000	$ 22,706,000	$ 119,829,000	$ 63,888,000	$ 63,100,000			$ 63,100,000