Fair Value Measurments - Debt Instruments (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	$ 3,430,000	$ 3,378,000
Notes Payable, Fair Value Disclosure	3,315,570	3,190,319
Notes Due 2015 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	0	100,000
Notes Payable, Fair Value Disclosure	$ 0	101,931
Debt 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	$ 207,952	203,738
Debt 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	$ 434,162	414,505
Debt 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	$ 64,670	65,027
Debt 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	$ 190,743	187,105
Debt 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	$ 117,959	116,240
Debt 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	$ 208,259	201,371
Debt 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	$ 85,564	82,581
Debt 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	$ 87,211	83,876
Debt 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	$ 294,866	283,335
Debt 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	$ 31,686	30,476
Debt Instruments Interest Rates	4.66%
Notes Due October 2024 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	$ 850,000	850,000
Notes Payable, Fair Value Disclosure	$ 768,994	754,485
Debt Instruments Interest Rates	6.125%
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	$ 153,504	147,649
Debt Instruments Interest Rates	4.91%
Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total outstanding debt	$ 670,000	518,000
Notes Payable, Fair Value Disclosure	$ 670,000	$ 518,000