Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Additional Information) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Senior Notes Three Point Five Zero Percent Due January Twenty Twenty Five [Member]
Debt Outstanding [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.50%
5.375% Senior Notes
Debt Outstanding [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.375%	5.375%
6.05% Subordinated Notes
Debt Outstanding [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.05%	6.05%
Fair value of the interest rate swap associated with the notes	4,300	4,600
7.0% Junior Subordinated Debentures
Debt Outstanding [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.00%	7.00%
Estimated Fair Value | 6.05% Subordinated Notes
Debt Outstanding [Line Items]
Fair value of the interest rate swap associated with the notes	4,300