Debt Commitments - Senior Unsecured Promissory Notes Payable (Details) - Senior Unsecured Promissory Notes	Dec. 31, 2018 USD ($) debt_instrument_series
Debt Instrument [Line Items]
Debt issuance, aggregate principal amount	$ 135,000,000
Maximum aggregate borrowing capacity	$ 600,000,000
Number of debt instrument Series | debt_instrument_series	3
2.00% Senior Unsecured Promissory Note Payable
Debt Instrument [Line Items]
Debt issuance, aggregate principal amount	$ 40,000,000
Fixed interest rate per annum	2.00%
2.45% Senior Unsecured Promissory Note Payable
Debt Instrument [Line Items]
Debt issuance, aggregate principal amount	$ 35,000,000
Fixed interest rate per annum	2.45%
3.22% Senior Unsecured Promissory Note Payable
Debt Instrument [Line Items]
Debt issuance, aggregate principal amount	$ 60,000,000
Fixed interest rate per annum	3.22%