Investment Information - Limited partnership interests (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Maximum Exposure to Loss		$ 1,423,044	$ 1,171,792
Aggregate unfunded commitments		1,630,000	1,700,000
Equity method investments
Schedule of Equity Method Investments [Line Items]
Maximum Exposure to Loss	[1]	1,292,573	1,041,321
Aggregate unfunded commitments		994,700	1,020,000
Fair value option
Schedule of Equity Method Investments [Line Items]
Maximum Exposure to Loss	[2]	130,471	130,471
Aggregate unfunded commitments		$ 91,000	$ 100,400

[1] Aggregate unfunded commitments were $994.7 million at June 30, 2018, compared to $1.02 billion at December 31, 2017.

[2] Aggregate unfunded commitments were $91.0 million at June 30, 2018, compared to $100.4 million at December 31, 2017.