Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Apr. 30, 2010
Financial assets (liabilities):
Restricted-use investment securities	$ 1,218	$ 513	$ 2,753
Carrying Amount
Financial assets (liabilities):
Restricted-use investment securities	1,218	513
Long-term debt (including current portion)	(1,405,206)	(957,236)
Fair Value
Financial assets (liabilities):
Restricted-use investment securities	1,218	513
Long-term debt (including current portion)	$ (1,410,255)	$ (951,306)