Investment Debt (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2014 Loan acquisition		Dec. 31, 2013
Debt Instrument [Line Items]
Long-term debt	$ 2,274.7		$ 850.8
Number of loans	19
Number of acquisitions partially financed with mortgages	8
Mortgage Loans and Notes Payable [Member]
Debt Instrument [Line Items]
Long-term debt	1,528.7
Debt unamortized premium	21.5
Mortgage Loans Payable [Member]
Debt Instrument [Line Items]
Long-term debt	1,528.7	[1]	401.8	[1]
Debt unamortized premium	21.5		5.3
Weighted average interest rate	3.68%		4.13%
Mortgage Loans Payable [Member] | Western United States [Member] | Mortgage Loans Secured by Multi-Family Properties [Member]
Debt Instrument [Line Items]
Long-term debt	349.8	[2]	261.0	[2]
Mortgage Loans Payable [Member] | Western United States [Member] | Mortgage Loans Secured by Commerical Buildings [Member]
Debt Instrument [Line Items]
Long-term debt	136.9		110.4
Mortgage Loans Payable [Member] | Western United States [Member] | Mortgage Loans Secured by Residential Buildings [Member]
Debt Instrument [Line Items]
Long-term debt	30.0		28.0
Mortgage Loans Payable [Member] | Japan [Member] | Mortgage Loans Secured by Multi-Family Properties [Member]
Debt Instrument [Line Items]
Long-term debt	283.7		0
Mortgage Loans Payable [Member] | Japan [Member] | Mortgage Loans Secured by Commerical Buildings [Member]
Debt Instrument [Line Items]
Long-term debt	2.4		2.4
Mortgage Loans Payable [Member] | Ireland [Member] | Mortgage Loans Secured by Multi-Family Properties [Member]
Debt Instrument [Line Items]
Long-term debt	200.6	[2]	0	[2]
Mortgage Loans Payable [Member] | Ireland [Member] | Mortgage Loans Secured by Commerical Buildings [Member]
Debt Instrument [Line Items]
Long-term debt	347.0	[2]	0	[2]
Mortgage Loans Payable [Member] | Ireland [Member] | Mortgage Loans Secured by Notes Receivable [Member]
Debt Instrument [Line Items]
Long-term debt	81.9	[2]	0	[2]
Mortgage Loans Payable [Member] | United Kingdom [Member] | Mortgage Loans Secured by Commerical Buildings [Member]
Debt Instrument [Line Items]
Long-term debt	$ 96.4	[2]	$ 0	[2]

[1] The investment debt had a weighted average interest rate of 3.68% and 4.13% per annum at June 30, 2014 and December 31, 2013.

[2] The investment debt balances include the unamortized debt premiums. Debt premiums represent the excess of the fair value of debt over the principal value of debt assumed in various acquisitions and are amortized into interest expense over the remaining term of the related debt in a manner that approximates the effective interest method. The unamortized loan premium as of June 30, 2014 and December 31, 2013 was $21.5 million and $5.3 million, respectively.