Long Term Debt	3 Months Ended
Mar. 31, 2018
Debt Disclosure [Abstract]
Long Term Debt	Long Term Debt Credit Facility The Company is party to a credit agreement that provides revolving credit commitments for up to $1.25 billion of borrowings, as well as term loan commitments, in each case maturing in January 2021. As of March 31, 2018, there was $135.0 million outstanding under the revolving credit facility and $155.0 million of term loan borrowings outstanding. At the Company's request and the lender's consent, revolving and or term loan borrowings may be increased by up to $300.0 million in aggregate, subject to certain conditions as set forth in the credit agreement, as amended. Incremental borrowings are uncommitted and the availability thereof will depend on market conditions at the time the Company seeks to incur such borrowings. The borrowings under the revolving credit facility have maturities of less than one year. Up to $50.0 million of the facility may be used for the issuance of letters of credit. There were $4.5 million of letters of credit outstanding as of March 31, 2018. The credit agreement contains negative covenants that, subject to significant exceptions, limit the ability of the Company and its subsidiaries to, among other things, incur additional indebtedness, make restricted payments, pledge their assets as security, make investments, loans, advances, guarantees and acquisitions, undergo fundamental changes and enter into transactions with affiliates. The Company is also required to maintain a ratio of consolidated EBITDA, as defined in the credit agreement, to consolidated interest expense of not less than 3.50 to 1.00 and is not permitted to allow the ratio of consolidated total indebtedness to consolidated EBITDA to be greater than 3.25 to 1.00 ("consolidated leverage ratio"). In February 2018, the Company amended the credit agreement to amend the definition of consolidated EBITDA, and to provide that the Company's trailing four-quarter consolidated leverage ratio may not exceed 3.75 to 1.00 for the four quarters ending June 30, 2018, and 4.00 to 1.00 for the four quarters ending September 30, 3018. Beginning with the four quarters ending December 31, 2018 and thereafter, the consolidated leverage ratio requirement will return to 3.25 to 1.00. As of March 31, 2018, the Company was in compliance with these ratios. In addition, the credit agreement contains events of default that are customary for a facility of this nature, and includes a cross default provision whereby an event of default under other material indebtedness, as defined in the credit agreement, will be considered an event of default under the credit agreement. Borrowings under the credit agreement bear interest at a rate per annum equal to, at the Company’s option, either (a) an alternate base rate, or (b) a rate based on the rates applicable for deposits in the interbank market for U.S. Dollars or the applicable currency in which the loans are made (“adjusted LIBOR”), plus in each case an applicable margin. The applicable margin for loans will be adjusted by reference to a grid (the “Pricing Grid”) based on the consolidated leverage ratio and ranges between 1.00% to 1.25% for adjusted LIBOR loans and 0.00% to 0.25% for alternate base rate loans. The weighted average interest rates under the outstanding term loans and revolving credit facility borrowings were 2.8% and 1.9% during the three months ended March 31, 2018 and 2017, respectively. The Company pays a commitment fee on the average daily unused amount of the revolving credit facility and certain fees with respect to letters of credit. As of March 31, 2018, the commitment fee was 15.0 basis points. Since inception, the Company incurred and deferred $3.9 million in financing costs in connection with the credit agreement. 3.250% Senior Notes In June 2016, the Company issued $600.0 million aggregate principal amount of 3.250% senior unsecured notes due June 15, 2026 (the “Notes”). The proceeds were used to pay down amounts outstanding under the revolving credit facility. Interest is payable semi-annually on June 15 and December 15 beginning December 15, 2016. Prior to March 15, 2026 (three months prior to the maturity date of the Notes), the Company may redeem some or all of the Notes at any time or from time to time at a redemption price equal to the greater of 100% of the principal amount of the Notes to be redeemed or a “make-whole” amount applicable to such Notes as described in the indenture governing the Notes, plus accrued and unpaid interest to, but excluding, the redemption date. On or after March 15, 2026 (three months prior to the maturity date of the Notes), the Company may redeem some or all of the Notes at any time or from time to time at a redemption price equal to 100% of the principal amount of the Notes to be redeemed, plus accrued and unpaid interest to, but excluding, the redemption date. The indenture governing the Notes contains covenants, including limitations that restrict the Company’s ability and the ability of certain of its subsidiaries to create or incur secured indebtedness and enter into sale and leaseback transactions and the Company’s ability to consolidate, merge or transfer all or substantially all of its properties or assets to another person, in each case subject to material exceptions described in the indenture. The Company incurred and deferred $5.3 million in financing costs in connection with the Notes. Other Long Term Debt In December 2012, the Company entered into a $50.0 million recourse loan collateralized by the land, buildings and tenant improvements comprising the Company's corporate headquarters. The loan has a seven year term and maturity date of December 2019. The loan bears interest at one month LIBOR plus a margin of 1.50%, and allows for prepayment without penalty. The loan includes covenants and events of default substantially consistent with the Company's credit agreement discussed above. The loan also requires prior approval of the lender for certain matters related to the property, including transfers of any interest in the property. As of March 31, 2018, December 31, 2017 and March 31, 2017, the outstanding balance on the loan was $39.5 million, $40.0 million and $41.5 million, respectively. The weighted average interest rate on the loan was 3.1% and 2.3% for the three months ended March 31, 2018 and 2017, respectively. Interest expense, net, was $8.6 million and $7.8 million for the three months ended March 31, 2018 and 2017, respectively. Interest expense includes the amortization of deferred financing costs, bank fees, capital and built-to-suit lease interest and interest expense under the credit and other long term debt facilities. The Company monitors the financial health and stability of its lenders under the credit and other long term debt facilities, however during any period of significant instability in the credit markets, lenders could be negatively impacted in their ability to perform under these facilities.

Long Term Debt

Credit Facility

The Company is party to a credit agreement that provides revolving credit commitments for up to $1.25 billion of borrowings, as well as term loan commitments, in each case maturing in January 2021. As of March 31, 2018, there was $135.0 million outstanding under the revolving credit facility and $155.0 million of term loan borrowings outstanding.

At the Company's request and the lender's consent, revolving and or term loan borrowings may be increased by up to $300.0 million in aggregate, subject to certain conditions as set forth in the credit agreement, as amended. Incremental borrowings are uncommitted and the availability thereof will depend on market conditions at the time the Company seeks to incur such borrowings.

The borrowings under the revolving credit facility have maturities of less than one year. Up to $50.0 million of the facility may be used for the issuance of letters of credit. There were $4.5 million of letters of credit outstanding as of March 31, 2018.

The credit agreement contains negative covenants that, subject to significant exceptions, limit the ability of the Company and its subsidiaries to, among other things, incur additional indebtedness, make restricted payments, pledge their assets as security, make investments, loans, advances, guarantees and acquisitions, undergo fundamental changes and enter into transactions with affiliates. The Company is also required to maintain a ratio of consolidated EBITDA, as defined in the credit agreement, to consolidated interest expense of not less than 3.50 to 1.00 and is not permitted to allow the ratio of consolidated total indebtedness to consolidated EBITDA to be greater than 3.25 to 1.00 ("consolidated leverage ratio"). In February 2018, the Company amended the credit agreement to amend the definition of consolidated EBITDA, and to provide that the Company's trailing four-quarter consolidated leverage ratio may not exceed 3.75 to 1.00 for the four quarters ending June 30, 2018, and 4.00 to 1.00 for the four quarters ending September 30, 3018. Beginning with the four quarters ending December 31, 2018 and thereafter, the consolidated leverage ratio requirement will return to 3.25 to 1.00. As of March 31, 2018, the Company was in compliance with these ratios. In addition, the credit agreement contains events of default that are customary for a facility of this nature, and includes a cross default provision whereby an event of default under other material indebtedness, as defined in the credit agreement, will be considered an event of default under the credit agreement.

Borrowings under the credit agreement bear interest at a rate per annum equal to, at the Company’s option, either (a) an alternate base rate, or (b) a rate based on the rates applicable for deposits in the interbank market for U.S. Dollars or the applicable currency in which the loans are made (“adjusted LIBOR”), plus in each case an applicable margin. The applicable margin for loans will be adjusted by reference to a grid (the “Pricing Grid”) based on the consolidated leverage ratio and ranges between 1.00% to 1.25% for adjusted LIBOR loans and 0.00% to 0.25% for alternate base rate loans. The weighted average interest rates under the outstanding term loans and revolving credit facility borrowings were 2.8% and 1.9% during the three months ended March 31, 2018 and 2017, respectively. The Company pays a commitment fee on the average daily unused amount of the revolving credit facility and certain fees with respect to letters of credit. As of March 31, 2018, the commitment fee was 15.0 basis points. Since inception, the Company incurred and deferred $3.9 million in financing costs in connection with the credit agreement.

3.250% Senior Notes

In June 2016, the Company issued $600.0 million aggregate principal amount of 3.250% senior unsecured notes due June 15, 2026 (the “Notes”). The proceeds were used to pay down amounts outstanding under the revolving credit facility. Interest is payable semi-annually on June 15 and December 15 beginning December 15, 2016. Prior to March 15, 2026 (three months prior to the maturity date of the Notes), the Company may redeem some or all of the Notes at any time or from time to time at a redemption price equal to the greater of 100% of the principal amount of the Notes to be redeemed or a “make-whole” amount applicable to such Notes as described in the indenture governing the Notes, plus accrued and unpaid interest to, but excluding, the redemption date. On or after March 15, 2026 (three months prior to the maturity date of the Notes), the Company may redeem some or all of the Notes at any time or from time to time at a redemption price equal to 100% of the principal amount of the Notes to be redeemed, plus accrued and unpaid interest to, but excluding, the redemption date.

The indenture governing the Notes contains covenants, including limitations that restrict the Company’s ability and the ability of certain of its subsidiaries to create or incur secured indebtedness and enter into sale and leaseback transactions and the Company’s ability to consolidate, merge or transfer all or substantially all of its properties or assets to another person, in each case subject to material exceptions described in the indenture. The Company incurred and deferred $5.3 million in financing costs in connection with the Notes.

Other Long Term Debt

In December 2012, the Company entered into a $50.0 million recourse loan collateralized by the land, buildings and tenant improvements comprising the Company's corporate headquarters. The loan has a seven year term and maturity date of December 2019. The loan bears interest at one month LIBOR plus a margin of 1.50%, and allows for prepayment without penalty. The loan includes covenants and events of default substantially consistent with the Company's credit agreement discussed above. The loan also requires prior approval of the lender for certain matters related to the property, including transfers of any interest in the property. As of March 31, 2018, December 31, 2017 and March 31, 2017, the outstanding balance on the loan was $39.5 million, $40.0 million and $41.5 million, respectively. The weighted average interest rate on the loan was 3.1% and 2.3% for the three months ended March 31, 2018 and 2017, respectively.

Interest expense, net, was $8.6 million and $7.8 million for the three months ended March 31, 2018 and 2017, respectively. Interest expense includes the amortization of deferred financing costs, bank fees, capital and built-to-suit lease interest and interest expense under the credit and other long term debt facilities.

The Company monitors the financial health and stability of its lenders under the credit and other long term debt facilities, however during any period of significant instability in the credit markets, lenders could be negatively impacted in their ability to perform under these facilities.