Investments (Mortgage Loans by Property Type) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Mortgage Loans on Real Estate [Line Items]
Recorded investment	$ 3,299,320	$ 3,136,764
Percentage of total	100.00%	100.00%
Apartment
Mortgage Loans on Real Estate [Line Items]
Recorded investment	$ 399,653	$ 420,014
Percentage of total	12.10%	13.40%
Retail
Mortgage Loans on Real Estate [Line Items]
Recorded investment	$ 1,011,124	$ 1,026,018
Percentage of total	30.60%	32.70%
Office building
Mortgage Loans on Real Estate [Line Items]
Recorded investment	$ 1,044,806	$ 980,858
Percentage of total	31.70%	31.30%
Industrial
Mortgage Loans on Real Estate [Line Items]
Recorded investment	$ 656,171	$ 527,485
Percentage of total	19.90%	16.80%
Other commercial
Mortgage Loans on Real Estate [Line Items]
Recorded investment	$ 187,566	$ 182,389
Percentage of total	5.70%	5.80%