Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Arrangements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reduction in Interest Income	$ (23,388)	$ (21,203)	$ (18,785)
Collaborative Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reduction in Interest Income	$ (1,200)	$ (1,200)	$ (1,100)
Minimum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Partnership Agreement Initial Term	2 years
Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Partnership Agreement Initial Term	10 years