Income Taxes Level 4 Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	$ 170	$ 133
Effective Income Tax Rate Reconciliation, Tax Exempt Income, Amount	(30)	(32)
Effective Income Tax Rate Reconciliation, Deduction, Dividends, Amount	(19)	(22)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount	0	(25)
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount	(5)	4
Income Tax Expense (Benefit)	109	58
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount		25
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions	0	0
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions	0	0
Unrecognized Tax Benefits	$ 12	12	$ 12	$ 12
Income Tax Examination, Description	The federal audit of the years 2012 and 2013 began in March 2015 and was completed as of March 31, 2017 with no additional adjustments. The Company has received notification from the Internal Revenue Service of its intention to audit the recently acquired subsidiary, Maxum, for its 2014 tax year. Management believes that adequate provision has been made in the consolidated financial statements for any potential adjustments that may result from tax examinations and other tax-related matters for all open tax years.
Stock-based Compensation [Member]
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, Deduction, Employee Stock Ownership Plan Dividend, Amount	$ (7)	$ 0