Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Jan. 29, 2015	Dec. 31, 2014
Debt Outstanding [Line Items]
Federal Home Loan Bank Advances	$ 60,000			$ 0
Short-term borrowings	77,766			7,781
Total long-term debt	802,917			453,443
Federal Home Loan Bank Advances
Debt Outstanding [Line Items]
Maturity	Apr. 01, 2015
Debt Instrument, Face Amount	60,000
Other short-term borrowings
Debt Outstanding [Line Items]
Debt Instrument, Face Amount	17,766	[1]
Short-term borrowings	17,766	[1]		7,781	[1]
Senior Notes Three Point Five Zero Percent Due January Twenty Twenty Five [Member]
Debt Outstanding [Line Items]
Maturity	Jan. 29, 2025
Debt Instrument, Face Amount	350,000		350,000
Senior Notes	349,710			0
5.375% Senior Notes
Debt Outstanding [Line Items]
Maturity	Sep. 15, 2020
Debt Instrument, Face Amount	350,000
Senior Notes	348,495			348,436
6.05% Subordinated Notes
Debt Outstanding [Line Items]
Maturity	Jun. 01, 2017	[2]
Debt Instrument, Face Amount	45,964	[2]
Subordinated Notes	49,910	[2]		50,162	[2]
7.0% Junior Subordinated Debentures
Debt Outstanding [Line Items]
Maturity	Oct. 15, 2033
Debt Instrument, Face Amount	50,000
Junior Subordinated Debentures	$ 54,802			$ 54,845

[1] Represents cash collateral received from certain counterparties in relation to market value exposures of derivative contracts in our favor, which includes an interest rate swap agreement related to our 6.05% Subordinated Notes.

[2] At March 31, 2015 and December 31, 2014, included in the carrying value of our 6.05% Subordinated Notes was an interest rate swap valued at $4.3 million and $4.6 million, respectively, related to hedge accounting associated with the notes.