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Short-Term Borrowings and Long-Term Debt - Additional Information (Detail) - USD ($)
1 Months Ended 3 Months Ended 12 Months Ended
Jan. 29, 2015
Sep. 30, 2010
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
May 15, 2007
Oct. 30, 2003
Debt Disclosure [Line Items]                              
Interest expense, debt                     $ 36,100,000 $ 37,300,000 $ 34,900,000    
Weighted average interest rates associated with short-term borrowings     1.39%       0.59%       1.39% 0.59%      
Short-term borrowings     $ 1,033,730,000       $ 512,668,000       $ 1,033,730,000 $ 512,668,000      
Proceeds from issuance of senior notes, net of discount and issuance cost                     0 0 346,431,000    
Interest expense     $ 11,310,000 $ 11,297,000 $ 11,231,000 $ 10,933,000 $ 10,728,000 $ 11,252,000 $ 10,656,000 $ 10,237,000 44,771,000 42,873,000 40,340,000    
Special Purpose Trust Entity                              
Debt Disclosure [Line Items]                              
Interest expense                     $ 3,300,000 $ 3,500,000 $ 3,500,000    
3.50% Senior Notes                              
Debt Disclosure [Line Items]                              
Stated interest rate     3.50%       3.50%       3.50% 3.50%      
Principal value $ 350,000,000   $ 350,000,000               $ 350,000,000        
Debt Instrument, Unamortized Discount     $ 300,000               $ 300,000        
Maturity                     Jan. 29, 2025        
Proceeds from issuance of senior notes, net of discount and issuance cost $ 346,400,000                            
5.375% Senior Notes                              
Debt Disclosure [Line Items]                              
Stated interest rate     5.375%       5.375%       5.375% 5.375%      
Principal value   $ 350,000,000 $ 350,000,000               $ 350,000,000        
Maturity                     Sep. 15, 2020        
Payments for settlement of 3.875% Convertible Notes   250,000,000                          
Proceeds from issuance of senior notes, net of discount and issuance cost   $ 345,000,000                          
6.05% Subordinated Notes                              
Debt Disclosure [Line Items]                              
Stated interest rate     6.05%       6.05%       6.05% 6.05%      
Principal value     $ 0               $ 0        
Debt Instrument, Unamortized Discount                           $ 250,000,000  
Maturity                     Jun. 01, 2017        
7.0% Junior Subordinated Debentures                              
Debt Disclosure [Line Items]                              
Stated interest rate     7.00%       7.00%       7.00% 7.00%      
Principal value     $ 0               $ 0       $ 50,000,000.0
Debt issuance costs                             $ 2,200,000
Federal Funds Purchased                              
Debt Disclosure [Line Items]                              
Principal value     330,000,000               $ 330,000,000        
Maturity                     Jan. 02, 2018        
Short-term borrowings     330,000,000       $ 0       $ 330,000,000 $ 0      
Federal Home Loan Bank Advances                              
Debt Disclosure [Line Items]                              
Principal value     700,000,000               $ 700,000,000        
Maturity                     Jan. 02, 2018        
Market value of collateral pledged     3,400,000,000               $ 3,400,000,000        
Unused and available funds     2,700,000,000               2,700,000,000        
Federal Reserve Bank Advances                              
Debt Disclosure [Line Items]                              
Market value of collateral pledged     $ 1,000,000,000               $ 1,000,000,000