Compensation Programs (Narrative) (Detail) (USD $)	12 Months Ended						12 Months Ended			1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Annual Incentive Compensation [Member] Y	Dec. 31, 2011 Annual Incentive Compensation [Member]	Dec. 31, 2010 Annual Incentive Compensation [Member]	Dec. 31, 2012 Other Benefit Programs [Member]	Dec. 31, 2011 Other Benefit Programs [Member]	Dec. 31, 2010 Other Benefit Programs [Member]	Jul. 31, 2012 2012 [Member]	Dec. 31, 2012 2012 [Member]	Dec. 31, 2012 Performance-Based Stock Awards Vesting In Three Year [Member]	Dec. 31, 2012 Performance-Based Stock Awards Vesting After Three Year [Member]	Dec. 31, 2012 Stock Options [Member] Y	Dec. 31, 2011 Stock Options [Member]	Dec. 31, 2010 Stock Options [Member]	Dec. 31, 2012 Annual Incentive Compensation [Member]	Dec. 31, 2011 Annual Incentive Compensation [Member]	Dec. 31, 2010 Annual Incentive Compensation [Member]	Dec. 31, 2012 Annual Incentive Compensation [Member] Executive Officer [Member]	Dec. 31, 2011 Annual Incentive Compensation [Member] Executive Officer [Member]	Dec. 31, 2010 Annual Incentive Compensation [Member] Executive Officer [Member]	Dec. 31, 2012 Long-Term Equity-Based Awards [Member] Y	Dec. 31, 2012 Stock Awards Activity [Member] Y	Dec. 31, 2011 Stock Awards Activity [Member]	Dec. 31, 2010 Stock Awards Activity [Member]
Share Based Compensation Programs [Line Items]
Total grants	654,000									72,000	72,000
Expiration date of grant	Jul. 01, 2015
Percentage of investment capital, maximum				10.00%	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%
Effective benchmark rate						8.00%
Incentive compensation expense																	$ 4,100,000	$ 5,700,000	$ 5,100,000
Incentive compensation award to executive officers																				156,251	189,283	164,829
Incentive compensation award per share value																				$ 11.47	$ 12.60	$ 12.71
Common shares for future issuance	528,018
Percentage of performance-based awards granted												50.00%	50.00%
Share awards vesting period (in years)													3 years	10 years									3 years
Share awards vesting period, minimum (in years)													1 year
Salaries and benefits	6,843,000	6,701,000	6,097,000				406,000	514,000	476,000						3,000	16,000								1,900,000	1,600,000	1,400,000
Other general and administrative expense																15,000								362,000	261,000	119,000
Common stock dividend payable	29,512,000	38,184,000																						765,000	333,000
Total of Unrecognized compensation expense for unvested stock award																								4,600,000
Weighted average remaining contractual term (in years)														4.7										1.6
Exercisable option awards outstanding														97,500
Weighted average exercise price														$ 11.63
Aggregate intrinsic value of stock options														54,000
Intrinsic value of option awards exercised														$ 620,000	$ 129,000	$ 3,000
Compensation plan of voluntary contribution							50.00%
Compensation plan of voluntary contribution, maximum							6.00%
Compensation plan of voluntary contribution, additional							3.00%