Subsequent Event (Narrative) (Details) - Subsequent Event [Member] $ in Millions	Apr. 17, 2018 USD ($)
Revolving Credit Facility [Member] | Minimum [Member]
Subsequent Event [Line Items]
Commitment fees percentage rate on unused portion of credit facility	0.10%
Revolving Credit Facility [Member] | Maximum [Member]
Subsequent Event [Line Items]
Commitment fees percentage rate on unused portion of credit facility	0.175%
Revolving Credit Facility [Member] | MUFG Union Bank, N.A. and Westpac Banking Corporation [Member]
Subsequent Event [Line Items]
Maximum borrowing capacity	$ 800
Uncommitted option to increase credit facility	300
Term Credit Agreement [Member] | MUFG Union Bank, N.A. and Westpac Banking Corporation [Member]
Subsequent Event [Line Items]
Maximum borrowing capacity	$ 200
Revolving Credit Facility And Term Credit Agreement [Member] | MUFG Union Bank, N.A. and Westpac Banking Corporation [Member]
Subsequent Event [Line Items]
Maturity date	Apr. 17, 2023
Principal payment	$ 6
Frequency of payments	semi-annual
London Interbank Offered Rate (LIBOR) [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Subsequent Event [Line Items]
Credit facility interest rate equal to reference rate plus	0.75%
London Interbank Offered Rate (LIBOR) [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Subsequent Event [Line Items]
Credit facility interest rate equal to reference rate plus	1.50%
Base Rate [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Subsequent Event [Line Items]
Credit facility interest rate equal to reference rate plus	0.00%
Base Rate [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Subsequent Event [Line Items]
Credit facility interest rate equal to reference rate plus	0.50%