Unsecured Lines Of Credit Payable - Narrative (Details)		9 Months Ended	12 Months Ended
	Jun. 23, 2015	Sep. 30, 2016	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jul. 21, 2016	Sep. 15, 2015
Line of Credit Facility [Line Items]
Total revolving credit facilities at December 31			$ 600,000,000	$ 600,000,000	$ 500,000,000
Weighted average interest rate during the year			1.52%	1.35%	1.53%
U.S. Covered Terrorism Losses, Percent			0.85
Insurance Provider, Covered Terrorism Losses, Percent			0.10
Aggregate Insurance Limit For Terrorism Losses			$ 100,000,000,000
Line of Credit [Member]
Line of Credit Facility [Line Items]
Weighted average interest rate during the year		1.00%
Line of Credit Facility, Commitment Fee Percentage		0.20%
Credit Facility No 1 [Member]
Line of Credit Facility [Line Items]
Total revolving credit facilities at December 31			100,000,000
Credit Facility No 2 [Member]
Line of Credit Facility [Line Items]
Total revolving credit facilities at December 31			400,000,000
credit facility 2015 [Member]
Line of Credit Facility [Line Items]
Total revolving credit facilities at December 31			1,000,000,000
credit facility 2015 [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Total revolving credit facilities at December 31			$ 600,000,000
line of credit facility, number of extensions allowed			2
Line of Credit Facility, extension period			6 months
credit facility 2015 [Member] | 2015 Term Loan [Member]
Line of Credit Facility [Line Items]
Long-term Line of Credit, Noncurrent			$ 150,000,000
Debt Instrument, Term			5 years 6 months
London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Variable Rate			0.77%
London Interbank Offered Rate (LIBOR) [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Spread on variable rate	1.00%
London Interbank Offered Rate (LIBOR) [Member] | 2015 Term Loan [Member]
Line of Credit Facility [Line Items]
Spread on variable rate			1.10%
Federal Funds Effective Swap Rate [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Spread on variable rate	0.50%
Interest Rate Swap [Member]
Line of Credit Facility [Line Items]
Derivative, Number of Instruments Held			2			2	2
Effective interest rate percentage			2.70%
Minimum [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Commitment Fee Percentage	0.125%
Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Spread on variable rate	0.875%
Minimum [Member] | Base Rate [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Spread on variable rate	0.00%
Maximum [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Commitment Fee Percentage	0.30%
Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Spread on variable rate	1.55%
Maximum [Member] | Base Rate [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Spread on variable rate	0.55%