Compensation Programs (Narrative) (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended	1 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended					3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Jul. 31, 2012 2012 [Member]	Sep. 30, 2012 2012 [Member]	Sep. 30, 2012 Other Benefit Programs [Member]	Sep. 30, 2012 Other Benefit Programs [Member]	Sep. 30, 2012 Annual Incentive Compensation [Member] Y	Sep. 30, 2012 Performance-Based Stock Awards Vesting In Three Year [Member]	Sep. 30, 2012 Performance-Based Stock Awards Vesting After Three Year [Member]	Sep. 30, 2012 Long-Term Equity-Based Awards [Member] Y	Sep. 30, 2012 Stock Options [Member]	Sep. 30, 2012 Stock Options [Member] Y	Sep. 30, 2012 Stock Awards Activity [Member]	Sep. 30, 2012 Stock Awards Activity [Member] Y
Share Based Compensation Programs [Line Items]
Total grants						72,000	72,000
Expiration date of grant			Jul. 01, 2015
Percentage of investment capital, maximum										10.00%			8.00%
Number of years for average long-term investment capital										10			10
Spread over 10-year U.S. Treasury rate (basis points)										2.00%			2.00%
Performance threshold multiplier, basis points										0.50%
Expenses related to compensation			$ 3,600,000		$ 5,700,000
Common shares for future issuance	829,668		829,668
Percentage of performance-based awards granted			50.00%								50.00%
Share awards vesting period (in years)			1 year									3 years	3 years		10 years
Salaries and benefits	1,696,000	1,631,000	5,205,000	5,004,000				86,000	336,000							469,000	1,406,000
Other general and administrative expense																94,000	255,000
Common stock dividend payable	36,173,000		36,173,000		38,184,000											643,000	643,000
Total of Unrecognized compensation expense for unvested stock award																3,500,000	3,500,000
Weighted average remaining contractual term (in years)															5.0		1.5
Exercisable option awards outstanding														97,500	97,500
Weighted average exercise price														$ 11.63	$ 11.63
Aggregate intrinsic value of stock options														190,000	190,000
Intrinsic value of option awards exercised														$ 495,000	$ 620,000
Compensation plan of voluntary contribution									50.00%
Compensation plan of voluntary contribution, maximum									6.00%
Compensation plan of voluntary contribution, additional									3.00%