Fair Value Measurements (Unobervable Quantitative Input Ratios) (Details) - Level 3 - Recurring - Real estate fund investments	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Minimum
Unobservable Quantitative Input
Discount rates	2.00%	10.00%
Capitalization rate	4.70%	4.30%
Maximum
Unobservable Quantitative Input
Discount rates	14.90%	14.90%
Capitalization rate	6.70%	5.80%
Weighted Average
Unobservable Quantitative Input
Discount rates	11.90%	12.60%
Capitalization rate	5.50%	5.30%