Credit Risk and Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Bank Credit Facility	$ 0	$ 270,000
Total fair value of debt instruments	3,140,819	3,101,760
June 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	314,001	314,618
October 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	282,813	279,375
November 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	207,040	201,500
November 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	178,125	176,438
January 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	446,378	445,188
May 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	897,813	884,094
April 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	230,660	216,500
July 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Payable, Fair Value Disclosure	0	310,000
March 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Bank Credit Facility	30,000	0
Notes Payable, Fair Value Disclosure	550,000	0
Mortgages [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Payable, Fair Value Disclosure	$ 3,989	$ 4,047