Equity and Capital Equity and Capital - Common Stock (Details)		1 Months Ended	3 Months Ended
	Feb. 16, 2017 USD ($)	Jan. 31, 2017 USD ($)	Mar. 31, 2017 USD ($) $ / shares shares	Mar. 31, 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			$ 151,457,000
Long-term Debt, Maturities, Repayments of Principal in Year Three			$ 230,653,000
Maximum [Member]
Class of Stock [Line Items]
Equity Issuances, Common Shares Authorized for Issuance | shares			200,000,000
Equity Offering, Common Shares Authorized for Issuance | shares			3,100,000
Forward Equity Offering, Common Shares available for issue | shares			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			$ 2,654,000	$ 2,453,000
Accumulated other comprehensive loss			$ (16,028,000)	(73,846,000)	$ (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
Accumulated Other Comprehensive Income (Loss) [Member]
Class of Stock [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			$ 2,610,000	2,417,000
Accumulated other comprehensive loss			$ (15,792,000)	$ (72,893,000)	(18,346,000)	$ (58,693,000)
ATM Equity Offering Program [Member]
Class of Stock [Line Items]
Shares issued | shares				182,787
Weighted Average Price Per Share | $ / shares				$ 68.85
Net proceeds from common stock issuance				$ 12,584,000
Payments for Commissions				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			(15,791,000)		$ (18,346,000)
Net proceeds from common stock issuance			$ 0	$ 12,293,000
Conversion of Stock, Conversion Ratio			0.45
Shares of common stock issued for merger (in shares) | shares			65,500,000
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
Stated rate on preferred stock			6.625%
Preferred Stock, Redemption Price Per Share | $ / shares			$ 25.21
Redemption value			$ 252,000,000
Notes Due February 1, 2047 [Member] | Senior Notes [Member]
Class of Stock [Line Items]
Term of debt		30 years
Face amount		$ 300,000,000