Reinsurance (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Net Amount	$ 4,410,647,000		$ 4,239,216,000		$ 3,733,005,000
Property and Casualty Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	1,517,000,000		1,372,200,000		1,161,500,000
Ceded to Other Companies	646,900,000	[1]	592,600,000	[1]	522,000,000	[1]
Assumed from Other Companies	3,540,500,000	[1]	3,459,600,000	[1]	3,093,500,000	[1]
Net Amount	$ 4,410,600,000		$ 4,239,200,000		$ 3,733,000,000
Percentage of Amount Assumed to Net	80.30%		81.60%		82.90%

[1] Include TransRe amounts starting from the TransRe Acquisition Date.