Notes Payable (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) instrument	Oct. 31, 2015 USD ($) derivative	Jun. 30, 2014 USD ($)	May. 31, 2014 USD ($) derivative	Apr. 30, 2013 USD ($) derivative	Jul. 31, 2011 USD ($) derivative	Sep. 30, 2007 USD ($)
Notes Payable [Line Items]
Number of derivative financial instruments outstanding | instrument	0
Debt Issuance Costs Incurred	$ 17,782,000
Interest Rate Swap
Notes Payable [Line Items]
Notional amount							$ 100,000,000
Liability amount from termination of hedge agreement							3,260,000
Liability amount from termination of hedge agreement, comprehensive income							3,228,000
2017 Notes
Notes Payable [Line Items]
Principal	250,000,000
Discount	877,000
Net Price	$ 249,123,000
Stated Rate	6.875%
Effective Rate	6.924%
2017 Notes | Interest Rate Swap
Notes Payable [Line Items]
Notional amount							100,000,000
Liability amount from termination of hedge agreement							3,260,000
Liability amount from termination of hedge agreement, comprehensive income							$ 3,228,000
2021 Notes
Notes Payable [Line Items]
Principal	$ 300,000,000
Discount	4,269,000
Net Price	$ 295,731,000
Stated Rate	5.50%
Effective Rate	5.689%
2021 Notes | Interest Rate Swap
Notes Payable [Line Items]
Notional amount						$ 150,000,000
Liability amount from termination of hedge agreement						5,300,000
Liability amount from termination of hedge agreement, comprehensive income						$ 5,218,000
Number of derivative financial instruments outstanding | derivative						2
2022 Notes
Notes Payable [Line Items]
Principal	$ 325,000,000
Discount	4,989,000
Net Price	$ 320,011,000
Stated Rate	3.80%
Effective Rate	3.985%
2023 Notes
Notes Payable [Line Items]
Principal	$ 350,000,000
Discount	2,594,000
Net Price	$ 347,406,000
Stated Rate	3.30%
Effective Rate	3.388%
2023 Notes | Forward Starting Swaps
Notes Payable [Line Items]
Notional amount					$ 240,000,000
Liability amount from termination of hedge agreement					3,156,000
Liability amount from termination of hedge agreement, comprehensive income					$ 3,141,000
Number of derivative financial instruments outstanding | derivative					4
2024 Notes
Notes Payable [Line Items]
Principal	$ 350,000,000
Discount	707,000
Net Price	$ 349,293,000
Stated Rate	3.90%
Effective Rate	3.924%
2024 Notes | Forward Starting Swaps
Notes Payable [Line Items]
Notional amount				$ 225,000,000
Liability amount from termination of hedge agreement				$ 6,312,000
Number of derivative financial instruments outstanding | derivative				3
2025 Notes
Notes Payable [Line Items]
Principal	$ 400,000,000
Discount	964,000
Net Price	$ 399,036,000
Stated Rate	4.00%
Effective Rate	4.029%
2025 Notes | Forward Starting Swaps
Notes Payable [Line Items]
Notional amount		$ 300,000
Liability amount from termination of hedge agreement		$ 13,369,000
Number of derivative financial instruments outstanding | derivative		4
2014 Notes
Notes Payable [Line Items]
Principal			$ 150,000,000
Stated Rate			6.25%
2015 Notes
Notes Payable [Line Items]
Principal	$ 150,000,000
Stated Rate	6.15%