Employee Benefit Plans (Summary Of Restricted Stock Units) (Details) - $ / shares	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Service-based Restricted Stock Units
Shares
Unvested, Beginning balance (in shares)			304,000
Granted (in shares)			0
Vested (in shares)			(105,000)
Forfeitures (in shares)			(35,000)
Unvested, Ending balance (in shares)			164,000	304,000
Weighted-Average Grant Date Fair Value
Unvested, balance outstanding (in dollars per share)			$ 61.79	$ 61.61
Granted (in dollars per share)			0.00
Vested (in dollars per share)			60.40
Forfeitures (in dollars per share)			$ 64.38
Vested (in shares)			(105,000)
Performance-Based Restricted Stock Units
Shares
Unvested, Beginning balance (in shares)			725,000
Granted (in shares)	343,000	263,000	0
Vested (in shares)			(257,000)
Forfeitures (in shares)			(268,000)
Unvested, Ending balance (in shares)			200,000	725,000
Weighted-Average Grant Date Fair Value
Unvested, balance outstanding (in dollars per share)			$ 59.76	$ 60.21
Granted (in dollars per share)			0.00
Vested (in dollars per share)			58.98
Forfeitures (in dollars per share)			$ 61.57
Performance vesting target achieved			72.00%	56.00%	81.00%
Vested (in shares)			(257,000)