Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2012	Dec. 31, 2011
Conversion of Stock [Line Items]
Cash	$ 52	$ 47	$ 50	$ 58
Alterra Common Stock [Member]
Conversion of Stock [Line Items]
Holdings of common stock and warrants, cost basis	177
Alterra Warrants [Member]
Conversion of Stock [Line Items]
Holdings of common stock and warrants, cost basis	63
Markel Corporation [Member]
Conversion of Stock [Line Items]
Transferred, common stock value	226
Cash	$ 98