Debt Obligations (Textual) (Details) - USD ($)							9 Months Ended	12 Months Ended
		Dec. 18, 2017	Nov. 16, 2017	Nov. 09, 2017	Oct. 08, 2015	May 12, 2015	Oct. 07, 2015	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015	Nov. 17, 2017	May 15, 2015
Debt Instrument [Line Items]
Loss on early extinguishment of debt								$ (3,933,000)		$ (66,590,000)		$ 0
Extinguishment of Debt, Amount								$ 325,000,000		0		$ 0
New Credit Facility [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate								2.28%
Debt term						4 years
Maximum borrowing capacity														$ 600,000,000
Increase in limit														400,000,000
Minimum Credit Spread on a LIBOR Rate Loan								0.00%
Maximum Credit Spread on a LIBOR Rate Loan								0.55%
Commitment fee percentage								0.25%
Line of credit								$ 0		$ 0
Interest expense borrowings								$ 2,600,000
Minimum fixed charge coverage ratio								1.5
Maximum leverage ratio								0.60
Maximum unsecured indebtedness to unencumbered asset value ratio								0.60
Maximum secured indebtedness to total asset value ratio								0.40
Minimum unencumbered cash flow to interest expense on unsecured debt ratio								1.75
Maximum percent of payments of dividends and distributions								95.00%
New Credit Facility [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Commitment fee percentage								0.125%
New Credit Facility [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Commitment fee percentage								0.30%
New Credit Facility [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity														65,000,000
New Credit Facility [Member] | Swing-Loans [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity														$ 60,000,000
New Credit Facility [Member] | LIBOR [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Variable interest rate								1.20%
Basis spread on variable rate								1.00%
New Credit Facility [Member] | LIBOR [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Variable interest rate								0.875%
New Credit Facility [Member] | LIBOR [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Variable interest rate								1.55%
New Credit Facility [Member] | Federal Funds Rate [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Basis spread on variable rate								0.50%
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate								4.04%		4.03%		5.72%
Long-term Debt, Gross								$ 320,107,000		$ 325,038,000
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross								1,628,610,000		1,703,610,000
Repayment of prepayment penalities			$ 1,200,000
Repayment of accrued interest			500,000
Unsecured Debt [Member] | $450.0M 3.95% Guaranteed Notes due 2027 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 450,000,000				$ 450,000,000	[1]	0	[1]
Debt Instrument, Interest Rate, Stated Percentage				3.95%
Debt instrument, Unamortized discount, Percent of face amount				99.25%
Debt instrument, Yield to maturity				4.04%
Debt instrument, Yield to maturity spread at time of pricing				1.70%
Treasury note rate term				10 years
Debt Instrument Maturity Date	[1]							Nov. 30, 2027
Effective interest rate	[1]							4.03%
Unsecured Debt [Member] | $250M 3.95% Guaranteed Notes due 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	[1]							$ 350,000,000		250,000,000
Debt Instrument, Interest Rate, Stated Percentage				3.95%
Debt Instrument, Face Amount				$ 100,000,000
Debt instrument, Unamortized discount, Percent of face amount				102.497%
Debt instrument, Yield to maturity				3.40%
Debt instrument, Yield to maturity spread at time of pricing				1.40%
Treasury note rate term				5 years
Debt Instrument Maturity Date	[1]							Feb. 15, 2023
Effective interest rate	[1]							3.87%
Unsecured Debt [Member] | Notes 2027 and 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount								$ 3,400,000
Debt Instrument, Unamortized Premium								2,500,000
Proceeds from Notes Payable				$ 546,600,000
Loss on early extinguishment of debt			(1,400,000)
Unsecured Debt [Member] | $325.0M 4.95% Guaranteed Notes due 2018 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	[1]							$ 0		325,000,000
Debt Instrument, Interest Rate, Stated Percentage													4.95%
Debt Instrument, Repurchased Face Amount		$ 211,600,000	115,100,000
Repayments of debt			$ 113,400,000
Repayment of prepayment penalities		2,300,000
Repayment of accrued interest		1,800,000
Loss on early extinguishment of debt		(2,500,000)
Debt instrument, cash redemption price		$ 215,700,000
Debt Instrument Maturity Date	[1]							Apr. 15, 2018
Effective interest rate	[1]							5.13%
Unsecured Debt [Member] | Seven Year Term Loan - Swapped to fixed [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross					$ 250,000,000		$ 200,000,000	$ 250,000,000		$ 250,000,000
Debt Instrument Maturity Date					Oct. 08, 2022		Feb. 01, 2019	Oct. 01, 2022
Increase in debt					$ 50,000,000
Increase in limit					$ 150,000,000
Effective interest rate					3.72%			3.72%
Unsecured Debt [Member] | Seven Year Term Loan - Swapped to fixed [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Variable interest rate					1.80%
Unsecured Debt [Member] | New Credit Facility [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity Date						May 15, 2019

[1] On November 17, 2017, the Company completed an offering of $450.0 million 3.95% Guaranteed Notes due 2027 (the “2027 Notes) and reopened the 3.95% Guaranteed Notes due 2023 (the “2023 Notes”) for an additional $100.0 million. On November 17, 2017, the Company redeemed the remaining outstanding balance of the 4.95% Guaranteed Notes due 2018 (the “2018 Notes”) ahead of its scheduled maturity. See below for further discussion.