Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Debt Outstanding [Line Items]
Interest Expense, Debt	$ 5,800,000		$ 11,600,000
Borrowings	5,808,000	5,817,000	11,600,000	11,611,000
Short-term borrowings	4,910,000		4,910,000		5,080,000
Total long-term debt	454,462,000		454,462,000		455,216,000
Other short-term borrowings
Debt Outstanding [Line Items]
Principal value	4,910,000		4,910,000
Short-term borrowings	4,910,000		4,910,000		5,080,000
5.375% Senior Notes
Debt Outstanding [Line Items]
Maturity			Sep. 15, 2020
Principal value	350,000,000		350,000,000
Senior Notes	348,321,000		348,321,000		348,209,000
6.05% Subordinated Notes
Debt Outstanding [Line Items]
Maturity			Jun. 01, 2017
Principal value	45,964,000		45,964,000
Subordinated Notes	51,208,000		51,208,000		51,987,000
7.0% Junior Subordinated Debentures
Debt Outstanding [Line Items]
Maturity			Oct. 15, 2033
Principal value	50,000,000		50,000,000
Junior Subordinated Debentures	$ 54,933,000		$ 54,933,000		$ 55,020,000