Derivative Financial Instruments Derivative Financial Instruments - Narrative (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) entity	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Potential recoveries under credit risk derivatives	$ 0
Gain (loss) on fair value hedge ineffectiveness, net	0	$ 0
Cash flow hedge gain to be reclassified from AOCI during the next twelve months	2,000,000
Cash and securities pledged as collateral by counterparties	4,000,000
Securities pledged as collateral to counterparties	7,000,000
Collateral already posted, aggregate fair value	4,000,000		$ 3,000,000
Collateral already posted, without credit-risk-contingent features	3,000,000
Securities pledged in the form of margin deposits	$ 3,000,000
The number of reference entities generally included in a CDX index | entity	125