Guarantees and Contingent Liabilities (Details)	9 Months Ended
	Sep. 30, 2018 USD ($) appeal
Proceedings:
Number of appeals | appeal	3
Minimum
Proceedings:
Loss contingencies, reasonably possible pretax loss exposure in excess of the amount accrued	$ 0
Maximum
Proceedings:
Loss contingencies, reasonably possible pretax loss exposure in excess of the amount accrued	115,000,000
Texas Windstorm Insurance Association
Proceedings:
Loss contingency, loss accrual	$ 37,000,000