Difference between Federal Income Tax Rate and Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reconciliation of Statutory Federal Tax Rate [Line Items]
Federal income tax rate	35.00%	35.00%	35.00%
Foreign tax credit and other adjustments	(0.90%)	(1.70%)
Income subject to dividends-received deduction	(4.00%)	(1.80%)	(0.80%)
Tax-exempt interest	(5.70%)	(4.30%)	(3.50%)
State taxes, net of federal tax benefit	0.40%	1.00%	0.40%
Other, net	0.10%	0.20%	0.40%
Effective income tax rate	24.90%	28.40%	31.50%