Investments - Fair Value Option (Details) - Fair Value Option [Member] $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, fair value disclosure	$ 106,151
Net Gains and Losses Recognized During the Period	560
Less Net Gains and Losses Recognized due to Sales	0
Total Changes in Fair Values Included in Current-Period Earnings	560
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, fair value disclosure	0
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, fair value disclosure	106,151
Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, fair value disclosure	$ 0