Credit Borrowings - Narrative (Details) € in Millions, £ in Millions				1 Months Ended	9 Months Ended
	Aug. 08, 2016 USD ($)	Jul. 25, 2016 USD ($)	Jul. 25, 2013 USD ($)	Oct. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 EUR (€)	Sep. 30, 2016 USD ($)	Sep. 30, 2016 GBP (£)	Dec. 31, 2015 EUR (€)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 GBP (£)	Feb. 04, 2015 EUR (€)	Feb. 04, 2015 GBP (£)
Line of Credit Facility [Line Items]
Credit facility								$ 671,023,000			$ 717,286,000
Partial repayments on credit facility					$ 42,136,000	$ 370,617,000
Carrying value of unencumbered assets								1,500,000,000
Bridge Loan Facility | Bridge Loan Facility
Line of Credit Facility [Line Items]
Bridge commitment	$ 150,000,000.0
Debt term	364 days
Commitment fee	1.50%
First 90 Days | Bridge Loan Facility | Bridge Loan Facility
Line of Credit Facility [Line Items]
Increase in margin rate	0.75%
Duration fee	1.00%
Every 90 Days Thereafter | Bridge Loan Facility | Bridge Loan Facility
Line of Credit Facility [Line Items]
Increase in margin rate	0.75%
180 Days After Signing | Bridge Loan Facility | Bridge Loan Facility
Line of Credit Facility [Line Items]
Commitment fee	0.375%
First 30 Days | Bridge Loan Facility | Bridge Loan Facility
Line of Credit Facility [Line Items]
Repayment fee on repayments	0.50%
30 Days Thereafter | Bridge Loan Facility | Bridge Loan Facility
Line of Credit Facility [Line Items]
Repayment fee on repayments	1.00%
LIBOR | Bridge Loan Facility | Bridge Loan Facility
Line of Credit Facility [Line Items]
Interest rate spread	3.25%
Maximum | Bridge Loan Facility | Bridge Loan Facility
Line of Credit Facility [Line Items]
Increase in margin rate	2.25%
Maximum | LIBOR | Bridge Loan Facility | Bridge Loan Facility
Line of Credit Facility [Line Items]
Effective Interest Rate	4.00%
Revolving Credit Facility
Line of Credit Facility [Line Items]
Credit facility													€ 110.5	£ 68.5
Revolving Credit Facility | JPMorgan Chase Bank, N.A.
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 50,000,000
Credit facility							€ 278.9	671,000,000	£ 160.2	€ 288.4	717,300,000	£ 160.2
Extension term		1 year
Extension fee		$ 1,500,000
Commitment fee percentage			50.00%
Fair value of credit facility								$ 671,000,000
Weighted average annual interest rate							2.40%	2.40%	2.40%
Remaining borrowing capacity								$ 69,000,000			$ 22,700,000
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Subsequent Event
Line of Credit Facility [Line Items]
Partial repayments on credit facility				$ 25,000,000
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Line of Credit Facility, Base Rate, Option Three
Line of Credit Facility [Line Items]
Interest rate spread			1.00%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Above Threshold
Line of Credit Facility [Line Items]
Unused capacity commitment fee			0.25%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Below Threshold
Line of Credit Facility [Line Items]
Unused capacity commitment fee			0.15%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Federal Funds | Line of Credit Facility, Base Rate, Option Two
Line of Credit Facility [Line Items]
Interest rate spread			0.50%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Maximum
Line of Credit Facility [Line Items]
Increase in additional borrowing capacity			$ 740,000,000
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Maximum | Base Rate | Line of Credit Facility, Interest Rate, Option Two
Line of Credit Facility [Line Items]
Interest rate spread			1.20%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Maximum | LIBOR | Line of Credit Facility, Interest Rate, Option One
Line of Credit Facility [Line Items]
Interest rate spread			2.20%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Minimum | Base Rate | Line of Credit Facility, Interest Rate, Option Two
Line of Credit Facility [Line Items]
Interest rate spread			0.60%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Minimum | LIBOR | Line of Credit Facility, Interest Rate, Option One
Line of Credit Facility [Line Items]
Interest rate spread			1.60%