Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012		Jun. 30, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 30,633		$ 15,462		$ 40,110
Restricted cash and equivalents	7,279		7,916		4,772
Notes payable	100,000		277,000		225,000
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	30,633		15,462		40,110
Restricted cash and equivalents	7,279		7,916		4,772
Notes payable	100,000		277,000		225,000
Long-term Debt	1,214,066		1,042,850		1,272,481
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value	30,633	[1]	15,462	[1]	40,110	[1]
Restricted Cash, Fair Value	7,279	[1]	7,916	[1]	4,772	[1]
Short-term Debt, Fair Value	100,000	[1]	277,000	[1]	225,000	[1]
Long-term Debt, Fair Value	$ 1,323,543	[2]	$ 1,231,559	[2]	$ 1,460,723	[2]

[1] Fair value approximates carrying value due to either the short-term length of maturity or variable interest rates that approximate prevailing market rates and therefore is classified in Level 1 in the fair value hierarchy.

[2] Long-term debt is valued based on observable inputs available either directly or indirectly for similar liabilities in active markets and therefore is classified in Level 2 in the fair value hierarchy.