Fair Value Measurments - Debt Instruments (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	$ 1,880,000	$ 1,837,000
Notes Due 2015 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	100,000	100,000
Notes Payable, Fair Value Disclosure	106,322	107,801
Private Debt Placement Instruments Interest Rates	5.45%
Notes Due 2018 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	200,000	200,000
Notes Payable, Fair Value Disclosure	221,832	230,062
Private Debt Placement Instruments Interest Rates	5.92%
Notes Due 2016 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	62,000	62,000
Notes Payable, Fair Value Disclosure	70,333	72,046
Private Debt Placement Instruments Interest Rates	7.31%
Notes Due 2019 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	173,000	173,000
Notes Payable, Fair Value Disclosure	208,642	219,045
Private Debt Placement Instruments Interest Rates	7.77%
Notes Due 2017 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	116,000	116,000
Notes Payable, Fair Value Disclosure	124,254	127,109
Private Debt Placement Instruments Interest Rates	4.98%
Notes Due January 2020 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	207,000	207,000
Notes Payable, Fair Value Disclosure	222,695	234,552
Private Debt Placement Instruments Interest Rates	5.50%
Notes Due January 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	87,000	87,000
Notes Payable, Fair Value Disclosure	92,293	98,526
Private Debt Placement Instruments Interest Rates	5.60%
Notes Due January 2025 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	90,000	90,000
Notes Payable, Fair Value Disclosure	95,017	102,096
Private Debt Placement Instruments Interest Rates	5.85%
Notes Due October 2020 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	315,000	315,000
Notes Payable, Fair Value Disclosure	312,738	331,329
Private Debt Placement Instruments Interest Rates	4.51%
Notes Due October 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	35,000	35,000
Notes Payable, Fair Value Disclosure	33,891	36,361
Private Debt Placement Instruments Interest Rates	4.66%
Notes Due October 2025 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	175,000	175,000
Notes Payable, Fair Value Disclosure	167,345	179,677
Private Debt Placement Instruments Interest Rates	4.91%
Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Excluding Current Maturities	320,000	277,000
Notes Payable, Fair Value Disclosure	$ 320,000	$ 277,000