Business Segments - Adjustments for Difference in Interim Medicare Part D Results for Segment Purposes and GAAP for Medicare Part D (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Disclosure [Line Items]
Benefit costs deferred	$ 527,340	$ 535,532	$ 1,711,530	$ 1,677,256
Government risk-sharing premium adjustment	$ (799,714)	$ (785,715)	(2,465,429)	(2,404,601)
Pre-tax addition to segment interim period income			20,971	16,850
After tax amount			13,631	10,952
Medicare Part D [Member]
Segment Reporting Disclosure [Line Items]
After tax amount			13,631	10,952
Adjustments [Member]
Segment Reporting Disclosure [Line Items]
Benefit costs deferred			11,649	53,561
Government risk-sharing premium adjustment			9,322	(28,532)
Adjustments [Member] | Medicare Part D [Member]
Segment Reporting Disclosure [Line Items]
Benefit costs deferred			11,649	45,382
Government risk-sharing premium adjustment			9,322	(28,532)
Pre-tax addition to segment interim period income			$ 20,971	$ 16,850