Debt Commitments - Senior Unsecured Promissory Notes Payable (Details) - Senior Unsecured Promissory Notes	Sep. 30, 2019 USD ($) debt_instrument_series
Debt Instrument [Line Items]
Debt issuance, aggregate principal amount	$ 135,000,000.0
Maximum aggregate borrowing capacity	$ 600,000,000.0
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	$ 40,000,000.0
Fixed interest rate per annum	2.00%
2.45% Senior Unsecured Promissory Note Payable
Debt Instrument [Line Items]
Debt issuance, aggregate principal amount	$ 35,000,000.0
Fixed interest rate per annum	2.45%
3.22% Senior Unsecured Promissory Note Payable
Debt Instrument [Line Items]
Debt issuance, aggregate principal amount	$ 60,000,000.0
Fixed interest rate per annum	3.22%