Debt - Long-Term Debt Maturities (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Mar. 31, 2013	Jun. 30, 2013	Jun. 30, 2013 Consolidated Entities [Member]	Jun. 30, 2013 Consolidated Entities [Member] Secured Mortgage Debt [Member]	Jun. 30, 2013 Consolidated Entities [Member] Secured Mortgage Debt [Member] Subsequent Event [Member]	Jun. 30, 2013 Consolidated Entities [Member] Credit Facilities [Member]	Jun. 30, 2013 Wholly Owned [Member]	Jun. 30, 2013 Wholly Owned [Member] Secured Mortgage Debt [Member]	Jun. 30, 2013 Wholly Owned [Member] Credit Facilities [Member]	Jun. 30, 2013 Wholly Owned [Member] Senior Notes [Member]	Jun. 30, 2013 Wholly Owned [Member] Exchangeable Notes [Member]	Jun. 30, 2013 Wholly Owned [Member] Other debt [Member]
Maturities Of Long Term Debt [Line Items]
2013		$ 175.0	$ 42.0	$ 34.7		$ 64.4	$ 133.0	$ 41.0		$ 92.0
2014		2,028	36				1,992	475	322	557		638
2015		1,119	25				1,094	149	309	175	460	1
2016		1,079	126				953	312		640		1
2017		969	4				965	264		700		1
2018		1,088	74				1,014	113		900		1
2019		944	2				942	294		647		1
2020		674	2				672	9		662		1
2021		138	2				136	135				1
2022		9	2				7	7
Thereafter		153	6				147	137				10
Subtotal		8,376	321				8,055	1,936	631	4,373	460	655
Unamortized premiums (discounts), net		41	1				40	39		64	(63)
Total		8,417	322				8,095	1,975	631	4,437	397	655
Repayment of debt	$ 1,400.0				$ 160.0