Summary Of Effective Income Tax Rate (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011		Jun. 30, 2012	Jun. 30, 2011
Reconciliation of Effective Income Tax Rate [Line Items]
Expected income taxes	$ 65,233	$ 73,168		$ 124,815	$ 124,799
Tax-exempt investment income	(869)	(847)		(1,737)	(1,693)
Low-income housing investments	(7,332)	(5,060)		(14,578)	(10,118)
Other	360	(990)		450	60
Income tax expense	$ 57,392	$ 66,271	[1]	$ 108,950	$ 113,048	[1]
Expected income taxes, percentage	35.00%	35.00%		35.00%	35.00%
Tax-exempt investment income, percentage	(0.50%)	(0.40%)		(0.50%)	(0.50%)
Low-income housing investments, percentage	(3.90%)	(2.40%)		(4.10%)	(2.80%)
Other, percentage	0.20%	(0.50%)		0.10%	0.00%
Income tax expense, percentage	30.80%	31.70%		30.50%	31.70%

[1] The 2011 balances have been retrospectively adjusted to give effect to the adoption of new accounting guidance as described in Note F-Adoption of New Accounting Standard.