Notes Payable (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) instrument	Oct. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) derivative	Dec. 31, 2016 USD ($) derivative	Oct. 31, 2015 USD ($) derivative	May 31, 2014 USD ($) derivative	Apr. 30, 2013 USD ($) derivative	Jul. 31, 2011 USD ($) derivative
Debt Instrument [Line Items]
Number of derivative financial instruments outstanding | instrument	0
Debt issuance costs incurred	$ 22,682,000
2021 Notes
Debt Instrument [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
Debt Instrument [Line Items]
Number of derivative financial instruments outstanding | derivative								2
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
2022 Notes
Debt Instrument [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
Debt Instrument [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
Debt Instrument [Line Items]
Number of derivative financial instruments outstanding | derivative							4
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
2024 Notes
Debt Instrument [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
Debt Instrument [Line Items]
Number of derivative financial instruments outstanding | derivative						3
Notional amount						$ 225,000,000
Liability amount from termination of hedge agreement						$ 6,312,000
2025 Notes
Debt Instrument [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
Debt Instrument [Line Items]
Number of derivative financial instruments outstanding | derivative					4
Notional amount					$ 300,000,000
Liability amount from termination of hedge agreement					$ 13,369,000
2026 Notes
Debt Instrument [Line Items]
Principal	$ 350,000,000
Discount	3,860,000
Net Price	$ 346,140,000
Stated Rate	3.60%
Effective Rate	3.733%
2026 Notes | Forward Starting Swaps
Debt Instrument [Line Items]
Number of derivative financial instruments outstanding | derivative				2
Notional amount				$ 180,000,000
Liability amount from termination of hedge agreement				$ 13,345,000
2027 Notes
Debt Instrument [Line Items]
Principal	$ 400,000,000
Discount	1,628,000
Net Price	$ 398,372,000
Stated Rate	3.50%
Effective Rate	3.548%
2027 Notes | Forward Starting Swaps
Debt Instrument [Line Items]
Number of derivative financial instruments outstanding | derivative			2
Notional amount			$ 250,000,000
Liability amount from termination of hedge agreement			7,690,000
Liability amount from termination of hedge agreement, comprehensive income			$ 7,688,000
2015 Notes
Debt Instrument [Line Items]
Principal		$ 250,000,000
Stated Rate		6.875%