Long-Term Debt - Components of Long-Term Debt (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Note	Dec. 31, 2013
Debt Instrument [Line Items]
Real Estate Loans and securities pledged	280	337
Senior Medium-Term Notes Due Through 2021 [Member]
Debt Instrument [Line Items]
Debt instrument maturity, year	2021
Long-term debt weighted average interest rate	3.89%	3.89%
Number of notes	3
0.975% Subordinated Notes Due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	0.975%
Debt instrument maturity, year	2028
6.875% Subordinated Notes Due 2029 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.875%
Debt instrument maturity, year	2029
7.750% Subordinated Notes Due 2029 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.75%
Debt instrument maturity, year	2029
Senior Medium-Term Notes Due Through 2039 [Member]
Debt Instrument [Line Items]
Debt instrument maturity, year	2039
Long-term debt weighted average interest rate	1.84%	1.58%
Number of notes	6
7.413% Subordinated Remarketable Notes Due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.413%
Debt instrument maturity, year	2027
5.80% Subordinated Notes Due 2014 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.80%
Debt instrument maturity, year	2014
4.95% Subordinated Notes Due 2015 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.95%
Debt instrument maturity, year	2015
5.45% Subordinated Notes Due 2016 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.45%
Debt instrument maturity, year	2016
5.70% Subordinated Notes Due 2017 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.70%
Debt instrument maturity, year	2017
4.625% Subordinated Notes Due 2018 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.625%
Debt instrument maturity, year	2018
6.95% Subordinated Notes Due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.95%
Debt instrument maturity, year	2028
Lease Financing Debt Due Through 2016 [Member]
Debt Instrument [Line Items]
Debt instrument maturity, year	2016
Long-term debt weighted average interest rate		5.99%
Secured Notes Due Through 2020 [Member]
Debt Instrument [Line Items]
Debt instrument maturity, year	2020
Federal Home Loan Bank [Member]
Debt Instrument [Line Items]
Debt instrument maturity, year	2036
Long-term debt weighted average interest rate	3.47%	3.47%
Investment Fund Financing [Member]
Debt Instrument [Line Items]
Debt instrument maturity, year	2052
Long-term debt weighted average interest rate	2.01%	2.01%
Secured Notes Due Through 2020 [Member]
Debt Instrument [Line Items]
Long-term debt weighted average interest rate	4.41%	4.79%
Redeemable One Month Prior to Maturity [Member] | Senior Medium-Term Notes Due Through 2021 [Member]
Debt Instrument [Line Items]
Number of notes	1
Redeemable One Month Prior to Maturity [Member] | Senior Medium-Term Notes Due Through 2039 [Member]
Debt Instrument [Line Items]
Number of notes	2
Non Redeemable Notes [Member] | Senior Medium-Term Notes Due Through 2021 [Member]
Debt Instrument [Line Items]
Number of notes	2
Non Redeemable Notes [Member] | Senior Medium-Term Notes Due Through 2039 [Member]
Debt Instrument [Line Items]
Number of notes	4