Variable Interest Entities - Summary of Assets and Liabilities Related to Consolidated Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Variable interest entities [Member]
Variable Interest Entity [Line Items]
Consolidated Assets	$ 380	$ 707
Consolidated Liabilities	167	185
Asset-backed commercial paper conduits [Member]
Variable Interest Entity [Line Items]
Consolidated Assets	0	250
Consolidated Liabilities	0	3
Low income housing limited liability partnership [Member]
Variable Interest Entity [Line Items]
Consolidated Assets	380	457
Consolidated Liabilities	167	182
Interest bearing deposits with banks [Member] | Asset-backed commercial paper conduits [Member]
Variable Interest Entity [Line Items]
Consolidated Assets	0	5
Securities held-to-maturity [Member] | Asset-backed commercial paper conduits [Member]
Variable Interest Entity [Line Items]
Consolidated Assets	0	200
Other assets [Member] | Asset-backed commercial paper conduits [Member]
Variable Interest Entity [Line Items]
Consolidated Assets	0	45
Other assets [Member] | Low income housing limited liability partnership [Member]
Variable Interest Entity [Line Items]
Consolidated Assets	380	457
Long-term debt [Member] | Low income housing limited liability partnership [Member]
Variable Interest Entity [Line Items]
Consolidated Liabilities	92	92
Interest, taxes and other liabilities [Member] | Asset-backed commercial paper conduits [Member]
Variable Interest Entity [Line Items]
Consolidated Liabilities	0	3
Interest, taxes and other liabilities [Member] | Low income housing limited liability partnership [Member]
Variable Interest Entity [Line Items]
Consolidated Liabilities	$ 75	$ 90