Debt Commitments and Contingencies - Unsecured Revolving Credit Facility (Details) - Credit Facility [Member] |
12 Months Ended |
---|---|
Dec. 31, 2016
USD ($)
| |
Debt Instrument [Line Items] | |
Weighted per annum interest rate on outstanding line of credit | 1.70% |
Minimum [Member] | |
Debt Instrument [Line Items] | |
Percentage fee paid for unused portion of credit facility | 0.10% |
Maximum [Member] | |
Debt Instrument [Line Items] | |
Percentage fee paid for unused portion of credit facility | 0.125% |
LIBOR [Member] | |
Debt Instrument [Line Items] | |
Per annum interest rate over LIBOR | 0.95% |
Unsecured Revolving Credit Facility [Member] | |
Debt Instrument [Line Items] | |
Credit facility, maximum borrowing capacity | $ 700,000,000 |
Letter of Credit Subfacility [Member] | |
Debt Instrument [Line Items] | |
Credit facility, maximum borrowing capacity | $ 55,000,000 |