Schedule I - Condensed Financial Information of Parent (Guarantees and Letters of Credit) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) agreement
Condensed Financial Statements, Captions [Line Items]
Obligations number of agreements | agreement	47
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 930,000,000
Parent Company
Condensed Financial Statements, Captions [Line Items]
Letter of credit, fee range minimum	0.25%
Letter of credit, fee range maximum	2.25%
Guarantees | Parent Company
Condensed Financial Statements, Captions [Line Items]
Obligations, Minimum Individual Exposures	$ 1,000,000
Obligations, individual exposures (minimum less than $1 million)	$ 272,000,000
Guarantees and Asset Sale Related Indemnities (Combined) [Member]
Condensed Financial Statements, Captions [Line Items]
Obligations number of agreements | agreement	22
Guarantees and Asset Sale Related Indemnities (Combined) [Member] | Parent Company
Condensed Financial Statements, Captions [Line Items]
Obligations maximum exposure, total	$ 842,000,000
Letter of Credit | Parent Company
Condensed Financial Statements, Captions [Line Items]
Obligations, individual exposures (minimum less than $1 million)	13,000,000
Cash Collateralized Letters Of Credit | Parent Company
Condensed Financial Statements, Captions [Line Items]
Obligations, Minimum Individual Exposures	2,000,000
Cash Collateralized Letters Of Credit | Parent Company | Maximum
Condensed Financial Statements, Captions [Line Items]
Obligations, individual exposures (minimum less than $1 million)	$ 26,000,000
Unsecured Debt [Member] | Financial Standby Letter of Credit [Member]
Condensed Financial Statements, Captions [Line Items]
Obligations number of agreements | agreement	4
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 52,000,000
Secured Debt [Member] | Standby Letters of Credit [Member]
Condensed Financial Statements, Captions [Line Items]
Obligations number of agreements | agreement	21
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 36,000,000