Fair Value Measurments - Debt Instruments (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	$ 2,437,000	$ 2,470,000
Notes Payable, Fair Value Disclosure	2,576,499	2,626,349
Notes Due 2015 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	100,000	100,000
Notes Payable, Fair Value Disclosure	103,950	105,913
Private Debt Placement Instruments Interest Rates	5.45%
Notes Due 2018 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	200,000	200,000
Notes Payable, Fair Value Disclosure	222,247	226,127
Private Debt Placement Instruments Interest Rates	5.92%
Notes Due December 2018 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	450,000	450,000
Notes Payable, Fair Value Disclosure	463,634	466,946
Private Debt Placement Instruments Interest Rates	5.75%
Notes Due 2016 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	62,000	62,000
Notes Payable, Fair Value Disclosure	68,371	70,228
Private Debt Placement Instruments Interest Rates	7.31%
Notes Due 2019 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	173,000	173,000
Notes Payable, Fair Value Disclosure	208,034	211,877
Private Debt Placement Instruments Interest Rates	7.77%
Notes Due 2017 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	116,000	116,000
Notes Payable, Fair Value Disclosure	123,818	126,342
Private Debt Placement Instruments Interest Rates	4.98%
Notes Due January 2020 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	207,000	207,000
Notes Payable, Fair Value Disclosure	226,818	229,068
Private Debt Placement Instruments Interest Rates	5.50%
Notes Due January 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	87,000	87,000
Notes Payable, Fair Value Disclosure	95,464	95,736
Private Debt Placement Instruments Interest Rates	5.60%
Notes Due January 2025 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	90,000	90,000
Notes Payable, Fair Value Disclosure	99,815	99,142
Private Debt Placement Instruments Interest Rates	5.85%
Notes Due October 2020 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	315,000	315,000
Notes Payable, Fair Value Disclosure	323,556	323,732
Private Debt Placement Instruments Interest Rates	4.51%
Notes Due October 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	35,000	35,000
Notes Payable, Fair Value Disclosure	35,703	35,494
Private Debt Placement Instruments Interest Rates	4.66%
Notes Due October 2025 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	175,000	175,000
Notes Payable, Fair Value Disclosure	178,089	175,744
Private Debt Placement Instruments Interest Rates	4.91%
Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	427,000	460,000
Notes Payable, Fair Value Disclosure	$ 427,000	$ 460,000