Shareholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Shareholders' Equity [Abstract]

Share Repurchase Activity

A summary of common share repurchases for the years ended December 31, 2017 and 2016 is as follows:

	Years Ended December 31,
(in millions, except per share data)	2017		2016
Common share repurchases, shares	9		10
Common share repurchases, average price per share	$	173.54	$	128.97
Common share repurchases, aggregate cost	$	1,500	$	1,280
Board authorized shares remaining	42		51