Notes Payable (Unsecured Senior Notes) (Details) - Senior Notes, unsecured	12 Months Ended
	Dec. 31, 2017 USD ($) tranche
Debt Instrument [Line Items]
Debt amount	$ 350,000,000
Number of tranches | tranche	2
Minimum shareholders' equity	$ 1,900,000,000.0
Minimum
Debt Instrument [Line Items]
Unencumbered interest coverage ratio	2.00
Fixed charge coverage ratio	1.50
Maximum
Debt Instrument [Line Items]
Leverage ratio (percent)	60.00%
10-year Note with Interest Rate of 4.09%
Debt Instrument [Line Items]
Debt amount	$ 100,000,000
Debt term	10 years
Interest rate (percent)	4.09%
8-year Note with Interest Rate of 3.91%
Debt Instrument [Line Items]
Debt amount	$ 250,000,000
Debt term	8 years
Interest rate (percent)	3.91%