Derivative Financial Instruments Derivative Financial Instruments - Narrative (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) entity	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Derivative Instruments, Gain (Loss)
Term of credit default swaps (in years)	5 years
Potential recoveries under credit risk derivatives	$ 0
Cash and securities pledged as collateral by counterparties	7,000,000
Cash and securities pledged as collateral to counterparties	1,000,000
Collateral already posted, aggregate fair value	0		$ 0
Securities pledged in the form of margin deposits	$ 8,000,000
The number of reference entities generally included in a CDX index | entity	125
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent
Derivative Instruments, Gain (Loss)
Gain (loss) on fair value hedge	$ 0	$ 2,000,000