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Fair Value Disclosures - Fair Value of Nonfinancial Assets (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Jan. 01, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]            
Impairment Charge $ 0 $ 234 $ 0 $ 267 $ 854  
Fair Value Net of Impairment 3,247,786   3,247,786   3,105,553 $ 3,056,236
Fair Value [Member] | Nonrecurring [Member]            
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]            
Fair Value Net of Impairment $ 0   $ 0   $ 12,950