Fair Value of Financial Instruments (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	$ 249,473	$ 233,897
Liabilities measured at fair value	2,674	1,793
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	0
Liabilities measured at fair value	0	$ 0
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	1		$ 1
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	1		1
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount	1		1
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount	$ 1		$ 1