Fair Value - Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financial assets:
Short-term investments	$ 59,474	$ 520,877
Financial liabilities:
Derivative instruments - interest rate swaps	65,078	22,124
Fair Value, Recurring | Carrying Amount
Financial assets:
Short-term investments	59,474	520,877
Fair Value, Recurring | Carrying Amount | Interest Rate Swap
Financial liabilities:
Derivative instruments - interest rate swaps	65,078	22,124
Fair Value, Recurring | Level 1 | Fair Value
Financial assets:
Short-term investments	0	0
Fair Value, Recurring | Level 1 | Fair Value | Interest Rate Swap
Financial liabilities:
Derivative instruments - interest rate swaps	0	0
Fair Value, Recurring | Level 2 | Fair Value
Financial assets:
Short-term investments	59,474	520,877
Fair Value, Recurring | Level 2 | Fair Value | Interest Rate Swap
Financial liabilities:
Derivative instruments - interest rate swaps	65,078	22,124
Fair Value, Recurring | Level 3 | Fair Value
Financial assets:
Short-term investments	0	0
Fair Value, Recurring | Level 3 | Fair Value | Interest Rate Swap
Financial liabilities:
Derivative instruments - interest rate swaps	$ 0	$ 0