Dividend restrictions - Insurance subsidiaries (Detail) (USD $) In Billions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Principal Insurance Subsidiaries [Member]
Statutory Accounting Practices [Line Items]
Amount available for payment of dividends without prior regulatory approval before end of 2013	$ 10.6
United States Based Property and Casualty Insurance Subsidiaries [Member]
Statutory Accounting Practices [Line Items]
Statutory shareholders' equity	$ 106	$ 95
Number of years for amortizing goodwill under statutory rules	10 years