Stock Option Plan and Stock Grants - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended							3 Months Ended	12 Months Ended				1 Months Ended				12 Months Ended			1 Months Ended													12 Months Ended									1 Months Ended							12 Months Ended					12 Months Ended	1 Months Ended
	Feb. 28, 2013	Jan. 31, 2013	May 08, 2012	May 11, 2011	Jan. 31, 2011	May 11, 2010	Feb. 28, 2010	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Maximum	Jan. 31, 2012 Director [Member]	Feb. 28, 2010 Director [Member]	Jan. 31, 2012 Senior Management	Feb. 28, 2010 Senior Management	Dec. 31, 2012 Restricted Stock [Member]	Dec. 31, 2011 Restricted Stock [Member]	Dec. 31, 2010 Restricted Stock [Member]	Jan. 31, 2013 Restricted Stock [Member] Director [Member]	May 08, 2012 Restricted Stock [Member] Director [Member]	Jan. 31, 2012 Restricted Stock [Member] Director [Member]	May 11, 2011 Restricted Stock [Member] Director [Member]	Jan. 31, 2011 Restricted Stock [Member] Director [Member]	May 11, 2010 Restricted Stock [Member] Director [Member]	Feb. 28, 2010 Restricted Stock [Member] Director [Member]	Feb. 28, 2013 Restricted Stock [Member] Senior Management	Jan. 31, 2012 Restricted Stock [Member] Senior Management	Feb. 28, 2011 Restricted Stock [Member] Senior Management	Feb. 28, 2010 Restricted Stock [Member] Senior Management	Dec. 31, 2012 One Third of Award	Dec. 31, 2012 Two Thirds of Award	Dec. 31, 2012 Restricted Stock [Member]	Dec. 31, 2011 Restricted Stock [Member]	Dec. 31, 2010 Restricted Stock [Member]	Dec. 31, 2009 Restricted Stock [Member]	Dec. 31, 2012 Common Stock	Dec. 31, 2011 Common Stock	Dec. 31, 2010 Common Stock	Dec. 31, 2011 Directors Plan Restricted Stock [Member]	Dec. 31, 2010 Directors Plan Restricted Stock [Member]	Jan. 31, 2013 Phase One [Member] Restricted Stock [Member] Director [Member]	May 08, 2012 Phase One [Member] Restricted Stock [Member] Director [Member]	Jan. 31, 2012 Phase One [Member] Restricted Stock [Member] Director [Member]	May 11, 2011 Phase One [Member] Restricted Stock [Member] Director [Member]	Jan. 31, 2011 Phase One [Member] Restricted Stock [Member] Director [Member]	May 11, 2010 Phase One [Member] Restricted Stock [Member] Director [Member]	Feb. 28, 2010 Phase One [Member] Restricted Stock [Member] Director [Member]	Dec. 31, 2012 Stock Option [Member]	Dec. 31, 2012 Stock Option [Member] Maximum	Dec. 31, 2012 Stock Option [Member] Vested One Third per Year	Dec. 31, 2012 Stock Option [Member] One Third of Award	Dec. 31, 2012 Stock Option [Member] Two Thirds of Award	Dec. 31, 2012 Restricted Stock [Member]	Jan. 31, 2013 Phase Two [Member] Restricted Stock [Member] Director [Member]	May 08, 2012 Phase Two [Member] Restricted Stock [Member] Director [Member]	Jan. 31, 2012 Phase Two [Member] Restricted Stock [Member] Director [Member]	May 11, 2011 Phase Two [Member] Restricted Stock [Member] Director [Member]	Jan. 31, 2011 Phase Two [Member] Restricted Stock [Member] Director [Member]	May 11, 2010 Phase Two [Member] Restricted Stock [Member] Director [Member]	Feb. 28, 2010 Phase Two [Member] Restricted Stock [Member] Director [Member]	Jan. 31, 2013 Phase Three [Member] Restricted Stock [Member] Director [Member]	May 08, 2012 Phase Three [Member] Restricted Stock [Member] Director [Member]	Jan. 31, 2012 Phase Three [Member] Restricted Stock [Member] Director [Member]	May 11, 2011 Phase Three [Member] Restricted Stock [Member] Director [Member]	Jan. 31, 2011 Phase Three [Member] Restricted Stock [Member] Director [Member]	May 11, 2010 Phase Three [Member] Restricted Stock [Member] Director [Member]	Feb. 28, 2010 Phase Three [Member] Restricted Stock [Member] Director [Member]	Jan. 31, 2013 Subsequent Event [Member] Restricted Stock [Member] Director [Member]	Feb. 28, 2013 Subsequent Event [Member] Restricted Stock [Member] Senior Management
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodNetOfForfeitures																																	47,010	47,010	52,343	56,008
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period																																	(88,999)	(113,665)	(125,330)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value																																	$ 64.60	$ 48.99	$ 44.19
Stock Issued During Period, Shares, Restricted Stock Award, Gross									107,332	119,665	121,665
Stock Issued During Period, Shares, Restricted Stock Award, Forfeited									(18,333)	(11,333)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value																																	$ 70.14	$ 57.40
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value									$ 1.7	$ 5.7	$ 0.8
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value									13.1	14.3	12.6
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period																					16,000	31,000	16,000	31,000	16,000	31,000		60,332	72,665	74,665																																							31,000	34,333
Share based compensation, grants awarded																																								47,000	47,000
Share based compensation, fair market value																				2.2	1.1	2.2	0.9	1.8	0.9	1.5	2.5	4.2	4.2	3.7
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights	These Restricted Stock Grants will vest on December 31, 2013.	One-third of the shares of restricted common stock covered by these awards vests on each of December 31, 2013, December 31, 2014, and December 31, 2015.	One-third of the shares of restricted common stock covered by these awards vests on each of November 8, 2012, May 8, 2013, and May 8, 2014.	One-third of the shares of restricted common stock covered by these awards vests on each of November 11, 2011, May 11, 2012, and May 11, 2013.	One-third of the shares of restricted common stock covered by these awards vests on each of December 31, 2011, December 31, 2012, and December 31, 2013.	One-third of the shares of restricted common stock covered by these awards vests on each of November 11, 2010, May 11, 2011, and May 11, 2012.	These Restricted Stock Grants vested December 31, 2011.						One-third of the shares of restricted common stock covered by these awards vests on each of December 31, 2012, December 31, 2013, and December 31, 2014.	One-third of the shares of restricted common stock covered by these awards vests on each of December 31, 2010, December 31, 2011, and December 31, 2012.	These Restricted Stock Grants vested on December 31, 2012.	These Restricted Stock Grants vested on December 31, 2010.																																	As to the Options that have been granted through December 31, 2012 to officers and employees, generally, one-third are exercisable one year after the initial grant, one-third are exercisable two years following the date such Options were granted and the remaining one-third are exercisable three years following the date such Options were granted.					Exercisability or vesting with respect to either type of award will be one-third of the award after six months, two-thirds of the award after one year, and the full award after two years.
Share based compensation, vesting period																																																		10 years	3 years
Share-based compensation, shares of common stock available for grant												6,000,000
Share-based compensation, options available for grant to any one individual												250,000
Share-based compensation share-based payment award, stock grant available for director																																																	10,000					2,000
Share-based compensation share-based payment award, stock option, vesting period																																																				1 year	2 years
Share-based compensation share-based payment award, restricted stock, vesting period									2 years																						6 months	1 year
Percentage of bonus that can be converted to common stock grant									50.00%
Share based compensation, restricted stock, vesting date																											Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010												Dec. 31, 2013	Nov. 08, 2012	Dec. 31, 2012	Nov. 11, 2011	Dec. 31, 2011	Nov. 11, 2010	Dec. 31, 2010							Dec. 31, 2014	May 08, 2013	Dec. 31, 2013	May 11, 2012	Dec. 31, 2012	May 11, 2011	Dec. 31, 2011	Dec. 31, 2015	May 08, 2014	Dec. 31, 2014	May 11, 2013	Dec. 31, 2013	May 11, 2012	Dec. 31, 2012
Share based compensation expense, restricted stock grant																	5.8	5.6	5.1
Share based compensation expense to be recognized																	2.8
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition																	1 year 6 months
Share based compensation, available for grants																	254,529	343,528	451,860																		654,346	743,345	851,677
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period								18,333			2,900
DeferredCompensationArrangementWithIndividualFairValueOfSharesCancelled								$ 1.3
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value																																	$ 70.12	$ 57.49	$ 50.03
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value																																	$ 65.93	$ 55.50	$ 37.22	$ 24.99