Investments - Loans by Property Type (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Loans by property type percentage of total loans	100.00%	100.00%
Loans Receivable, Gross, Commercial, Real Estate	$ 3,666.1	$ 3,514.1
Industrial
Investment Holdings [Line Items]
Loans by property type percentage of total loans	20.70%	22.70%
Loans Receivable, Gross, Commercial, Real Estate	758.6	796.8
Retail
Investment Holdings [Line Items]
Loans by property type percentage of total loans	35.30%	35.20%
Loans Receivable, Gross, Commercial, Real Estate	1,292.4	1,236.4
Office
Investment Holdings [Line Items]
Loans by property type percentage of total loans	12.40%	12.60%
Loans Receivable, Gross, Commercial, Real Estate	455.4	443.1
Apartments
Investment Holdings [Line Items]
Loans by property type percentage of total loans	15.80%	15.70%
Loans Receivable, Gross, Commercial, Real Estate	581.0	550.6
Hotel/Motel
Investment Holdings [Line Items]
Loans by property type percentage of total loans	4.10%	4.20%
Loans Receivable, Gross, Commercial, Real Estate	151.3	149.7
Mixed Use
Investment Holdings [Line Items]
Loans by property type percentage of total loans	3.40%	4.10%
Loans Receivable, Gross, Commercial, Real Estate	124.4	142.8
Other
Investment Holdings [Line Items]
Loans by property type percentage of total loans	8.30%	5.50%
Loans Receivable, Gross, Commercial, Real Estate	$ 303.0	$ 194.7