Variable Interest Entities (Details)	12 Months Ended
Dec. 31, 2009 Agreement Company	Mar. 31, 2013 Minimum [Member]	Mar. 31, 2013 Maximum [Member]	Mar. 31, 2013 Unconsolidated Variable Interest Entities [Member] Minimum [Member]	Mar. 31, 2013 Unconsolidated Variable Interest Entities [Member] Maximum [Member]
Noncontrolling Interest [Line Items]
Percentage equity interest (in hundredths)		20.00%	58.20%	20.00%	50.00%
Number of separate agreements with unrelated third parties	2
Number of limited liabilities formed	2

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

No definition available.

Name:	us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant