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Related Party Transactions - Additional information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Related Party Transactions (Textual) [Abstract]      
IT service payments to NMIC and NSC $ 361 $ 324 $ 277
Company made lease payments to NMIC 16 17 19
Total account values of group annuity and life insurance contracts 3,400 3,400  
Revenue from group annuity and life insurance contracts 119 125 127
Total interest credited to the account balances 107 111 111
Revenues ceded to NMIC [1] 444 329 369
Customer allocations to NFG funds 61,600 66,700 61,400
NFG paid to NLIC 230 221 199
Amounts on deposit with NCMC 1,400 1,000  
Commercial mortgage loans 12,417 10,963  
Other investments 1,891 1,493  
Total commissions and fees paid 72 72 65
Notes receivable outstanding 321 313 332
Nation Wide Bank [Member]      
Related Party Transactions (Textual) [Abstract]      
Fixed-rate advances 1,000    
Affiliate of the Company, were transferred to the Company along with of cash 772    
Commercial mortgage loans 207    
Fixed maturity securities 152    
Other investments 65    
Nationwide Trust Company [Member]      
Related Party Transactions (Textual) [Abstract]      
Notes receivable outstanding $ 165 0 0
Interest rate 0.785%    
Receivable due date 2019-03    
Additional Paid in Capital $ 435    
Nationwide Advantage Mortgage Company [Member]      
Related Party Transactions (Textual) [Abstract]      
Revenues ceded to NMIC 257 158 209
Notes receivable outstanding $ 4 7 11
Interest rate 5.57%    
Receivable due date 2019-11    
Minimum [Member]      
Related Party Transactions (Textual) [Abstract]      
Interest rate 3.27%    
Receivable due date 2022-01    
Maximum [Member]      
Related Party Transactions (Textual) [Abstract]      
Interest rate 5.00%    
Receivable due date 2038-06    
Nationwide Mutual Insurance Company [Member]      
Related Party Transactions (Textual) [Abstract]      
Revenues ceded to NMIC $ 257 158 209
Benefits, claims and expenses $ 237 $ 108 $ 185
[1] Amount includes revenues ceded to NMIC of $257 million, $158 million and $209 million for the years ended December 31, 2018, 2017 and 2016, respectively, under a modified coinsurance agreement whereby all of the Company's accident and health business not ceded to unaffiliated reinsurers is ceded to NMIC.