Stock Compensation Plans - RSU, PRS and Cash RSU Activity (Detail) shares in Thousands	12 Months Ended
	Dec. 31, 2018 $ / shares shares
RSU's [Member]
Number of Shares
Beginning balance (in shares) | shares	441
Granted (in shares) | shares	169
Vested (in shares) | shares	(145)
Forfeited (in shares) | shares	(17)
Ending balance (in shares) | shares	448
Weighted Average Grant Date Fair Value Per Share
Beginning balance (in dollars per share) | $ / shares	$ 119.45
Granted (in dollars per share) | $ / shares	132.70
Vested (in dollars per share) | $ / shares	113.89
Forfeited (in dollars per share) | $ / shares	125.30
Ending balance (in dollars per share) | $ / shares	$ 125.99
Restricted Stock [Member]
Number of Shares
Beginning balance (in shares) | shares	159
Granted (in shares) | shares	67
Vested (in shares) | shares	(61)
Forfeited (in shares) | shares	(3)
Ending balance (in shares) | shares	162
Weighted Average Grant Date Fair Value Per Share
Beginning balance (in dollars per share) | $ / shares	$ 124.15
Granted (in dollars per share) | $ / shares	140.10
Vested (in dollars per share) | $ / shares	118.05
Forfeited (in dollars per share) | $ / shares	129.77
Ending balance (in dollars per share) | $ / shares	$ 132.96
Cash RSU's [Member]
Number of Shares
Beginning balance (in shares) | shares	95
Granted (in shares) | shares	31
Vested (in shares) | shares	(30)
Forfeited (in shares) | shares	(4)
Ending balance (in shares) | shares	92
Weighted Average Grant Date Fair Value Per Share
Beginning balance (in dollars per share) | $ / shares	$ 152.61
Granted (in dollars per share) | $ / shares	132.23
Vested (in dollars per share) | $ / shares	125.38
Forfeited (in dollars per share) | $ / shares	137.83
Ending balance (in dollars per share) | $ / shares	$ 132.23