Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Long-term debt, carrying value	$ 1,299,105	$ 1,300,000
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Long-term debt fair value	1,300,000	1,400,000
Senior notes | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Long-term debt fair value	1,000,000	1,000,000
Mortgage loans | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Long-term debt fair value	$ 236,200
Mortgages and Line of Credit | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Long-term debt fair value		$ 364,600