Fair Value Measurements (Unobervable Quantitative Input Ratios) (Details) - Recurring - Level 3 - Real estate fund investments	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Minimum
Unobservable Quantitative Input
Discount rates	10.00%	2.00%
Capitalization rate	4.70%	4.70%
Maximum
Unobservable Quantitative Input
Discount rates	15.10%	14.90%
Capitalization rate	5.80%	6.70%
Weighted Average
Unobservable Quantitative Input
Discount rates	12.60%	11.90%
Capitalization rate	5.60%	5.50%