Equity and Capital Equity and Capital - Common Stock (Details)		1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 16, 2017 USD ($)	Jan. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Class of Stock [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Two			$ 150,617,000		$ 150,617,000
Long-term Debt, Maturities, Repayments of Principal in Year Three			$ 42,076,000		42,076,000
Partners' Capital Account, Contributions					13,467,000	$ 8,675,000
Payments of Stock Issuance Costs					$ 349,000	80,000
Maximum [Member]
Class of Stock [Line Items]
Equity Issuances, Common Shares Authorized for Issuance | shares			500,000,000		500,000,000
Equity Offering, Common Shares Authorized for Issuance | shares			3,100,000		3,100,000
Forward Equity Offering, Common Shares available for issue | shares			1,250,000		1,250,000
AOCI Including Portion Attributable to Noncontrolling Interest [Member]
Class of Stock [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			$ 2,329,000	$ 43,111,000	$ 8,054,000	48,063,000
Accumulated other comprehensive loss			$ (14,453,000)	(36,663,000)	$ (14,453,000)	(36,663,000)	$ (18,647,000)	$ (59,478,000)
Common Stock [Member]
Class of Stock [Line Items]
Forward Equity Offering, Agreement Price Per Share, Common Stock | $ / shares			$ 75.25		$ 75.25
Accumulated Other Comprehensive Income (Loss) [Member]
Class of Stock [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					$ 7,922,000	47,880,000
Accumulated other comprehensive loss			$ (14,141,000)	$ (35,739,000)	$ (14,141,000)	$ (35,739,000)	(18,346,000)	$ (58,693,000)
ATM Equity Offering Program [Member]
Class of Stock [Line Items]
Shares issued | shares					0	182,787
Weighted Average Price Per Share | $ / shares					$ 0.00	$ 68.85
Net proceeds from common stock issuance					$ 0	$ 12,584,000
Payments for Commissions					$ 0	157,000
Forward Equity Offering [Member]
Class of Stock [Line Items]
Shares issued | shares					1,850,000
Net proceeds from common stock issuance					$ 137,500,000
Parent Company [Member]
Class of Stock [Line Items]
Accumulated other comprehensive loss			(14,141,000)		(14,141,000)		$ (18,346,000)
Net proceeds from common stock issuance					$ 0	549,545,000
Conversion of Stock, Conversion Ratio					0.45
Shares of common stock issued for merger (in shares) | shares					65,500,000
Partners' Capital Account, Contributions					$ 13,467,000	8,675,000
Parent Company [Member] | Common Stock [Member]
Class of Stock [Line Items]
Partners' Capital Account, Contributions					0	0
Parent Company [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Class of Stock [Line Items]
Partners' Capital Account, Contributions					$ 0	0
Parent Company [Member] | Noncontrolling Interest Exchangeable Operating Partnership Units [Member]
Class of Stock [Line Items]
Partners' Capital Account, Units, Contributed | shares					195,732
Partners' Capital Account, Contributions					$ 13,100,000	$ 0
Series 6 [Member]
Class of Stock [Line Items]
Stated rate on preferred stock	6.625%
Redemption value	$ 250,000,000
Series 6 [Member] | Parent Company [Member]
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference, Value			$ 250,000,000		$ 250,000,000
Stated rate on preferred stock					6.625%
Preferred Stock, Redemption Price Per Share | $ / shares			$ 25.21		$ 25.21
Redemption value			$ 252,000,000		$ 252,000,000
Series 7 [Member] | Parent Company [Member]
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference, Value			$ 75,000,000		$ 75,000,000
Stated rate on preferred stock					6.00%
Preferred Stock, Redemption Price Per Share | $ / shares			$ 25.22		$ 25.22
Redemption value			$ 75,700,000.0		$ 75,700,000.0
Notes Due February 1, 2047 [Member] | Senior Notes [Member]
Class of Stock [Line Items]
Term of debt		30 years
Face amount		$ 300,000,000