Derivative Financial Instruments (Master Netting Schedule) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative asset, Fair Value	$ 21	$ 52
Derivative liability, Fair Value	19
Gross Amounts Offset in the Statement of Financial Position, Assets	1	1
Gross Amounts Offset in the Statement of Financial Position, Liabilities	0
Financial Instruments, Assets	0	0
Financial Instruments, Liabilities	0
Derivative Asset, Collateral, Obligation to Return Cash, Offset	10	36
Cash Collateral, Liabilities	12
Derivative asset	10	15
Net Amount, Liabilities	7
Other assets
Derivative [Line Items]
Derivative asset, Fair Value	57	46
Net Amount Presented in the Statement of Financial Position	20	51
Other liabilities
Derivative [Line Items]
Derivative liability, Fair Value	18	20
Net Amount Presented in the Statement of Financial Position	19
Swap futures
Derivative [Line Items]
Derivative asset, Fair Value	4
Gross Amounts Offset in the Statement of Financial Position, Assets	1
Financial Instruments, Assets	0
Derivative Asset, Collateral, Obligation to Return Cash, Offset	0
Derivative asset	3
Swap futures | Other assets
Derivative [Line Items]
Derivative asset, Fair Value	3
Net Amount Presented in the Statement of Financial Position	3
Swap futures | Other liabilities
Derivative [Line Items]
Derivative liability, Fair Value	1
U.S. Treasury and euro dollar futures
Derivative [Line Items]
Derivative asset, Fair Value	10	18
Gross Amounts Offset in the Statement of Financial Position, Assets	0	1
Financial Instruments, Assets	0	0
Derivative Asset, Collateral, Obligation to Return Cash, Offset	9	10
Derivative asset	1	7
U.S. Treasury and euro dollar futures | Other assets
Derivative [Line Items]
Derivative asset, Fair Value	1	7
Net Amount Presented in the Statement of Financial Position	10	17
U.S. Treasury and euro dollar futures | Other liabilities
Derivative [Line Items]
Derivative liability, Fair Value	0	1
Mortgage backed securities forwards
Derivative [Line Items]
Derivative asset, Fair Value		26
Derivative liability, Fair Value	13
Gross Amounts Offset in the Statement of Financial Position, Assets		0
Gross Amounts Offset in the Statement of Financial Position, Liabilities	0
Financial Instruments, Assets		0
Financial Instruments, Liabilities	0
Derivative Asset, Collateral, Obligation to Return Cash, Offset		24
Cash Collateral, Liabilities	12
Derivative asset		2
Net Amount, Liabilities	1
Mortgage backed securities forwards | Other assets
Derivative [Line Items]
Derivative asset, Fair Value	0	2
Net Amount Presented in the Statement of Financial Position		26
Mortgage backed securities forwards | Other liabilities
Derivative [Line Items]
Derivative liability, Fair Value	1	6
Net Amount Presented in the Statement of Financial Position	13
Interest rate swaps
Derivative [Line Items]
Derivative asset, Fair Value	7	8
Derivative liability, Fair Value	6	6
Gross Amounts Offset in the Statement of Financial Position, Assets	0	0
Gross Amounts Offset in the Statement of Financial Position, Liabilities	0	0
Financial Instruments, Assets	0	0
Financial Instruments, Liabilities	0	0
Derivative Asset, Collateral, Obligation to Return Cash, Offset	1	2
Cash Collateral, Liabilities	0	0
Derivative asset	6	6
Net Amount, Liabilities	6	6
Interest rate swaps | Other assets
Derivative [Line Items]
Derivative asset, Fair Value	6	6
Net Amount Presented in the Statement of Financial Position	7	8
Interest rate swaps | Other liabilities
Derivative [Line Items]
Derivative liability, Fair Value	6
Net Amount Presented in the Statement of Financial Position	$ 6	$ 6