Legal and Regulatory Proceedings - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2009	Mar. 31, 2013
Commitments and Contingencies Disclosure
Number of putative securities class action complaints		$ 5
Number of cases that have been consolidated		5
Number of additional lawsuit filed		2
Number of alleging violations of ERISA that were dismissed		2
Amount in excess of amounts reserved			73,000,000
Payments for Legal Settlements	16,000,000
Class Settlement Escrow	46,000,000
Non Class Action Escrow	$ 4,000,000