Debt - Long-Term Debt Maturities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Maturities Of Long Term Debt [Line Items]
2013	$ 800.0
2014	2,310
2015	847
2016	1,080
2017	969
2018	1,087
2019	946
2020	679
2021	143
2022	9
Thereafter	153
Subtotal	9,023
Unamortized premiums (discounts), net	51
Total	9,074
Wholly Owned [Member]
Maturities Of Long Term Debt [Line Items]
2013	683.0
2014	2,231
2015	796
2016	951
2017	930
2018	1,014
2019	944
2020	677
2021	141
2022	7
Thereafter	147
Subtotal	8,521
Unamortized premiums (discounts), net	49
Total	8,570
Wholly Owned [Member] | Senior Notes [Member]
Maturities Of Long Term Debt [Line Items]
2013	299.0
2014	895
2015	175
2016	640
2017	700
2018	900
2019	647
2020	667
Subtotal	4,923
Unamortized premiums (discounts), net	70
Total	4,993
Wholly Owned [Member] | Exchangeable Notes [Member]
Maturities Of Long Term Debt [Line Items]
2013	342.0
2015	460
Subtotal	802
Unamortized premiums (discounts), net	(64)
Total	738
Wholly Owned [Member] | Credit Facilities [Member]
Maturities Of Long Term Debt [Line Items]
2014	383
2015	38
Subtotal	421
Total	421
Wholly Owned [Member] | Other Debt [Member]
Maturities Of Long Term Debt [Line Items]
2014	526
2015	1
2016	1
2017	1
2018	1
2019	1
2020	1
2021	1
Thereafter	10
Subtotal	543
Total	543
Wholly Owned [Member] | Secured Mortgage Debt [Member]
Maturities Of Long Term Debt [Line Items]
2013	42.0
2014	427
2015	122
2016	310
2017	229
2018	113
2019	296
2020	9
2021	140
2022	7
Thereafter	137
Subtotal	1,832
Unamortized premiums (discounts), net	43
Total	1,875
Consolidated Entities [Member]
Maturities Of Long Term Debt [Line Items]
2013	117.0
2014	79
2015	51
2016	129
2017	39
2018	73
2019	2
2020	2
2021	2
2022	2
Thereafter	6
Subtotal	502
Unamortized premiums (discounts), net	2
Total	504
Consolidated Entities [Member] | Credit Facilities [Member]
Maturities Of Long Term Debt [Line Items]
2013	64.4
Consolidated Entities [Member] | Secured Mortgage Debt [Member]
Maturities Of Long Term Debt [Line Items]
2013	$ 34.7