XML 117 R80.htm IDEA: XBRL DOCUMENT v3.8.0.1
Income Taxes - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Taxes [Line Items]      
Tax at statutory federal rate 35.00% 35.00% 35.00%
Tax cuts and jobs act, reduction in income tax expense related to revaluing deferred tax assets and liabilities $ 99,500,000    
Federal tax benefits resulting from investments in renewable energy tax credit funds 52,400,000 $ 62,200,000 $ 1,900,000
Tax cuts and jobs act, provisional tax amount related to deductibility of executive compensation expense 4,500,000    
Deferred tax assets valuation allowance 0 0  
Net taxes payable 23,800,000 41,200,000  
Provision for income taxes 540,800,000 413,500,000 611,100,000
Interest and Penalties [Member]      
Income Taxes [Line Items]      
Provision for income taxes $ 200,000 $ 100,000 $ 100,000
ARX and subsidiaries      
Income Taxes [Line Items]      
Noncontrolling interest, ownership percentage (less than) 80.00%    
Net unrealized gains (losses) on securities      
Income Taxes [Line Items]      
Tax cuts and jobs act, reduction in income tax expense related to revaluing deferred tax liability associated with net unrealized gains on investment portfolio $ 275,700,000    
Other Deferred Tax Assets and Liabilities [Member]      
Income Taxes [Line Items]      
Tax cuts and jobs act, reduction in income tax expense related to other deferred tax assets (liabilities) $ 176,200,000    
Tax Year 2018      
Income Taxes [Line Items]      
Tax at statutory federal rate 21.00%