Investments - Equity Securities (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Net gains (losses) recognized during the period on equity securities	$ 2,109	$ (13,789,000)
Less: Net gains recognized during the period on equity securities sold during the period	0	6,764,000
Unrealized gains (losses) recognized during the period on equity securities still held at the end of the period	$ 2,109	$ (20,553,000)