Fair Value (Nonrecurring Fair Value Measurements) (Details) (Non Recurring [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Real Estate Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value Prior to Measurement	$ 0	$ 10	$ 5	$ 15
Gain (Losses)	0	(2)	(2)	(5)
Real Estate Funds [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value After Measurement	0	8	3	10
Held-for-investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value Prior to Measurement	232	197	225	197
Gain (Losses)	10	12	17	13
Held-for-investments [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value After Measurement	242	209	242	210
Held-for-sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value Prior to Measurement	0	254	0	270
Gain (Losses)	0	(15)	0	(31)
Held-for-sale [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value After Measurement	0	239	0	239
Other limited partnership interests [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value Prior to Measurement	108	38	109	48
Gain (Losses)	(39)	(11)	(39)	(14)
Other limited partnership interests [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value After Measurement	$ 69	$ 27	$ 70	$ 34