Fair Value Measurements - Investments in Company-Sponsored Funds (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Investment Holdings [Line Items]
Fair Value	$ 7,617
Unfunded Commitments	1,400
Hedge Funds [Member]
Investment Holdings [Line Items]
Fair Value	2,618	[1]
Unfunded Commitments	$ 0	[1]
Redemption Frequency	Quarterly - Annually	[1]
Private Equity Funds
Investment Holdings [Line Items]
Investments Lock In Period	10 years
Fair Value	$ 4,999	[2]
Unfunded Commitments	$ 1,400	[2]

[1] Includes investments in hedge funds and hedge fund of funds that pursue long/short, event-driven, and activist strategies. Each hedge fund has various restrictions regarding redemption; no investment is locked-up for a period greater than one year.

[2] Includes private equity funds and private equity fund of funds with a focus on diversified portfolios, real estate and global natural resources. Due to the illiquid nature of these funds, investors are not permitted to make withdrawals without the consent of the general partner. The lock-up period of the private equity funds can extend to 10 years.