Stock-Based Compensation Plans (Narrative) (Detail) (USD $)	3 Months Ended											3 Months Ended						3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Performance Shares [Member]	Dec. 31, 2012 Performance Shares [Member]	Mar. 31, 2012 Performance Shares [Member]	Dec. 31, 2011 Performance Shares [Member]	Mar. 31, 2013 Performance Shares [Member] Maximum [Member]	Mar. 31, 2013 Restricted Stock [Member]	Dec. 31, 2012 Restricted Stock [Member]	Mar. 31, 2012 Restricted Stock [Member]	Dec. 31, 2011 Restricted Stock [Member]	Mar. 31, 2013 Restricted Stock [Member] Minimum [Member]	Mar. 31, 2013 Restricted Stock [Member] Maximum [Member]	Mar. 31, 2013 Nineteen Ninety Seven Plan [Member]	Mar. 31, 2013 Two Thousand And Seven Plan [Member]	May 31, 2011 Two Thousand And Seven Plan [Member]	May 31, 2009 Two Thousand And Seven Plan [Member]	Mar. 31, 2013 Two Thousand And Seven Plan [Member] Minimum [Member]	Mar. 31, 2013 Two Thousand And Seven Plan [Member] Maximum [Member]	Mar. 31, 2013 Ltip Awards [Member]	Mar. 31, 2013 Ltip Awards [Member] Minimum [Member]	Mar. 31, 2013 Ltip Awards [Member] Maximum [Member]	Mar. 31, 2013 Federal Funds Rate [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Borrowings Agreement Considered Base Rate Loans on Bear Interest Rate	4.00%																						1.00%
Allocated Share-based Compensation Expense	$ 2,300,000	$ 2,300,000
Shares approved for issuance															500,000
Shares additionally approved for issuance																2,860,000	325,000
Shares available for future grants															686,256
Award vesting period, maximum, years												1 year	5 years					3 years	5 years	3 years
Share based payment award options term														10 years	7 years					7 years
Percentage of performance based award payouts																					0.00%	200.00%
Weighted average grant date fair value per share of options granted	$ 17.59	$ 14.91
Aggregate intrinsic value of options exercised	$ 102,000	$ 4,500,000
Maximum number of performance-vested shares that could be issued			254,290	153,915	185,616	72,158	629,000	203,827	314,226	326,776	336,709