Long-term Debt - Borrowings (Details) (USD $)			12 Months Ended	1 Months Ended		1 Months Ended		1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011 Revolving Credit Facility Unsecured Five Year Revolving Credit Facility 2011	Feb. 29, 2012 Revolving Credit Facility Unsecured Five Year Revolving Credit Facility 2012	Dec. 31, 2012 Revolving Credit Facility Unsecured Five Year Revolving Credit Facility 2012	Feb. 29, 2012 Bridge Loan Bridge Credit Facility, Settlement of RSU's from IPO	Feb. 29, 2012 Bridge Loan Bridge Credit Facility, Settlement of RSU's from IPO 180 days after borrowing	Feb. 29, 2012 Bridge Loan Bridge Credit Facility, Settlement of RSU's from IPO 90 after borrowing	Oct. 25, 2012 Unsecured debt Amended and Restated Term Loan	Oct. 25, 2012 Unsecured debt Unsecured Term Loan
Debt Instrument
Line Of Credit Maturity Period			5 years	5 years
Term loan facility, term period									3 years
Line of credit facility, maximum borrowing capacity			$ 2,500,000,000	$ 5,000,000,000		$ 3,000,000,000
Debt instrument, interest rate during period			LIBOR	LIBOR		LIBOR			LIBOR
Basis spread on variable rate			1.00%	1.00%		1.00%			1.00%
Debt Instrument, Unused Capacity, Commitment Fee Percentage									0.10%
Long-term debt	1,500,000,000	0							1,500,000,000
Line of credit facility, amount outstanding			0		0
Line of credit facility, unused capacity, commitment fee percentage				0.10%				0.10%
Debt Instrument, Face Amount										$ 1,500,000,000
Line of credit facility, used capactiy, additional fee percentage							0.25%
Term loan facility upfront fee percentage									0.15%