Investment Information - Other investments (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		$ 2,328,904	$ 2,401,370
Available for sale
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	[1]	0	264,989
Available for sale | Asian and emerging markets
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		0	135,140
Available for sale | Investment grade fixed income
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		0	53,878
Available for sale | Credit related funds
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		0	18,365
Available for sale | Other
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		0	57,606
Fair value option
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		2,328,904	2,136,381
Fair value option | Term loan investments
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		1,286,305	1,200,882
Other investments par		1,321,154	1,223,453
Fair value option | Mezzanine debt funds
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		257,750	252,160
Fair value option | Asian and emerging markets
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		350,308	258,541
Fair value option | Investment grade fixed income
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		98,240	102,347
Fair value option | Credit related funds
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		199,437	175,422
Fair value option | Other
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	[2]	$ 136,864	$ 147,029

[1] The Company reviewed the accounting treatment for three limited partnership investments which were accounted for as available for sale at December 31, 2017 during the 2018 first quarter and determined, based on reconsideration during the period of the Company’s percentage ownership, that the equity method of accounting was appropriate for such investments.

[2] Includes fund investments with strategies in mortgage servicing rights, transportation, infrastructure assets and other.