Derivative Financial Instruments - Schedule of Interest Rate Derivatives (Details) - Interest rate swap - Fair Value Hedges	Sep. 30, 2018
Senior notes | 2.55% notes due 2020
Derivative [Line Items]
Fixed interest rate	2.55%
Senior notes | 4.38% note due 2021
Derivative [Line Items]
Fixed interest rate	4.38%
Senior notes | 1.95% Notes Due 2021
Derivative [Line Items]
Fixed interest rate	1.95%
Senior notes | 2.65% Notes Due 2022
Derivative [Line Items]
Fixed interest rate	2.65%
Senior notes | 3.7% notes due in 2023
Derivative [Line Items]
Fixed interest rate	3.70%
Senior notes | 3.3% notes due 2024
Derivative [Line Items]
Fixed interest rate	3.30%
Senior notes | 3.55% notes due 2025
Derivative [Line Items]
Fixed interest rate	3.55%
Senior notes | 2.65% notes due 2026
Derivative [Line Items]
Fixed interest rate	2.65%
Subordinated note | 3.10% subordinated note due 2023
Derivative [Line Items]
Fixed interest rate	3.10%