Commitments and Contingencies (Details) (USD $)	12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012
Commitments and Contingencies
Term for which office lease can be extended	5 years
Future minimum lease payments
2015	$ 11,837,000
2016	8,937,000
2017	8,483,000
2018	6,142,000
2019	5,512,000
2020 and thereafter	183,151,000
Total minimum lease payments	224,062,000
Total minimum rentals to be received in the future under non-cancelable subleases	2,600,000
Sublease recoveries	900,000
Lease and rental expense, net of sublease recoveries	11,900,000	11,200,000	14,900,000
Contingent rent included in lease and rental expense	400,000	300,000	700,000
Unconditional purchase obligations
2015	2,783,323,000
2016	278,380,000
2017	65,873,000
2018	241,193,000
2019	1,477,000
Total future purchase commitments	3,370,246,000
Forward physical and financial sales for all future periods	3,361,100,000
Issued and outstanding letter of credit to counterparties to support natural gas purchase commitments	$ 4,800,000