Short-Term Debt	12 Months Ended
Dec. 31, 2018
Debt Disclosure [Abstract]
Short-Term Debt	(11) Short-Term Debt The Company classifies debt as short-term if the maturity date at inception is less than one year. In December 2015, the Company renewed an agreement to increase its $600 million commercial paper program to $750 million. The Company had $362 million and no amount outstanding under this agreement as of December 31, 2018 and 2017, respectively. In March 2018, the Company renewed an agreement with the FHLB to extend its ability to borrow in order to provide financing for operations. This extension, which expires on March 22, 2019, allows the Company access to borrow up to $250 million, which would be collateralized by pledged securities, at a borrowing rate set by the FHLB. The Company had $5.8 billion and $6.5 billion in eligible collateral and no amounts outstanding under the agreement as of December 31, 2018 and 2017, respectively. In April 2015, NMIC and the Company entered into a $750 million credit facility with a borrowing rate of 0.785% plus a U.S. LIBOR rate, which is based on the repayment date of the draw, which expires on April 2, 2020. The Company had no amounts outstanding under this agreement as of December 31, 2018 and 2017. The Company has entered into an agreement with its custodial bank to borrow against the cash collateral that is posted in connection with its securities lending program. The maximum amount available under the agreement is $350 million. The borrowing rate on this program is equal to the one-month U.S. LIBOR. The Company had no amounts outstanding under this agreement as of December 31, 2018 and 2017. In December 2018, the Company entered into an agreement with NFS to borrow $45 million at a borrowing rate of 3.55% due in March 2019. The amount of interest paid on short-term debt was immaterial for the years ended December 31, 2018, 2017 and 2016.

(11) Short-Term Debt

The Company classifies debt as short-term if the maturity date at inception is less than one year.

In December 2015, the Company renewed an agreement to increase its $600 million commercial paper program to $750 million. The Company had $362 million and no amount outstanding under this agreement as of December 31, 2018 and 2017, respectively.

In March 2018, the Company renewed an agreement with the FHLB to extend its ability to borrow in order to provide financing for operations. This extension, which expires on March 22, 2019, allows the Company access to borrow up to $250 million, which would be collateralized by pledged securities, at a borrowing rate set by the FHLB. The Company had $5.8 billion and $6.5 billion in eligible collateral and no amounts outstanding under the agreement as of December 31, 2018 and 2017, respectively.

In April 2015, NMIC and the Company entered into a $750 million credit facility with a borrowing rate of 0.785% plus a U.S. LIBOR rate, which is based on the repayment date of the draw, which expires on April 2, 2020. The Company had no amounts outstanding under this agreement as of December 31, 2018 and 2017.

The Company has entered into an agreement with its custodial bank to borrow against the cash collateral that is posted in connection with its securities lending program. The maximum amount available under the agreement is $350 million. The borrowing rate on this program is equal to the one-month U.S. LIBOR. The Company had no amounts outstanding under this agreement as of December 31, 2018 and 2017.

In December 2018, the Company entered into an agreement with NFS to borrow $45 million at a borrowing rate of 3.55% due in March 2019.

The amount of interest paid on short-term debt was immaterial for the years ended December 31, 2018, 2017 and 2016.