Debt	12 Months Ended
Dec. 31, 2015
Debt Instruments [Abstract]
Debt	Debt On March 28, 2013, the Company issued two series of senior notes with an aggregate principal amount of $700,000 (the “2013 Senior Notes”). The Company received net proceeds of $698,093 from this transaction, which represents the principal amount less the discount before offering expenses. The discount of $1,907 is being amortized over the life of the 2013 Senior Notes and is included as part of interest expense on the consolidated statements of operations. The first series is $350,000 in principal amount, bears interest at 2.50% per year and is payable in a single installment due March 15, 2018 and was issued at a 0.18% discount. The second series is $350,000 in principal amount, bears interest at 4.00% per year and is payable in a single installment due March 15, 2023 and was issued at a 0.37% discount. Interest on the 2013 Senior Notes is payable semi-annually on March 15 and September 15 of each year. The 2013 Senior Notes are unsecured obligations and rank equally with all of the Company’s other senior unsecured indebtedness. The Company may redeem each series of the 2013 Senior Notes in whole or in part at any time and from time to time before their maturity at the redemption price set forth in the Indenture. The 2013 Senior Notes are registered under the Securities Act of 1933, as amended. The interest expense incurred related to the 2013 Senior Notes was $22,988, $22,981 and $17,357 for the year ended December 31, 2015, 2014 and 2013, respectively. There was $6,635 of accrued interest at both December 31, 2015 and 2014. The Company made interest payments on the 2013 Senior Notes of $11,375 on March 15, 2015 and 2014 and September 15, 2015 and 2014. In February 2004, the Company issued two series of senior notes with an aggregate principal amount of $975,000 (the “2004 Senior Notes”). The Company received net proceeds of $971,537 from this transaction, which represents the principal amount less the discount before operating expenses. The discount of $3,463 is being amortized over the life of the 2004 Senior Notes and is included as part of interest expense on the statement of operations. The first series was $500,000 in principal amount, issued at a 0.11% discount, bore interest at 5.63% per year and was repaid on February 18, 2014. The second series is $475,000 in principal amount, bears interest at 6.75% per year and is payable in a single installment due February 15, 2034 and was issued at a 0.61% discount. Interest on the 2004 Senior Notes is payable semi-annually on February 15 and August 15 of each year. The 2004 Senior Notes are unsecured obligations and rank equally with all of the Company’s other senior unsecured indebtedness. The senior notes are not redeemable prior to maturity. All of the holders of the 2004 Senior Notes exchanged their notes in May 2004 for new notes registered under the Securities Act of 1933, as amended. The interest expense incurred related to the 2004 Senior Notes was $32,127, $35,414, and $59,414 for the years ended December 31, 2015, 2014, and 2013, respectively. There was $12,023 of accrued interest at both December 31, 2015 and 2014. The Company made interest payments on the 2004 Senior Notes of $16,031 and $30,094 on February 15, 2015 and 2014, respectively, and $16,031 on August 15, 2015 and 2014. Credit Facility The Company’s commercial paper program requires the Company to maintain liquidity facilities either in an available amount equal to any outstanding notes from the commercial paper program or in an amount sufficient to maintain the ratings assigned to the notes issued from the commercial paper program. The Company’s subsidiaries do not maintain commercial paper or other borrowing facilities at their level. This program is currently backed up by a $400,000 senior revolving credit facility, of which $395,960 was available at December 31, 2015, due to $4,040 of outstanding letters of credit related to this program. On September 16, 2014, the Company entered into a five-year unsecured $400,000 revolving credit agreement, as amended by Amendment No. 1, dated as of March 5, 2015 (the “2014 Credit Facility”) with a syndicate of banks arranged by JP Morgan Chase Bank, N.A. and Wells Fargo, N.A. The 2014 Credit Facility replaces the Company's prior four-year $350,000 revolving credit facility (the "2011 Credit Facility"), which was entered into on September 21, 2011 and was scheduled to expire in September 2015. The 2011 Credit Facility terminated upon the effectiveness of the 2014 Credit Facility. The 2014 Credit Facility provides for revolving loans and the issuance of multi-bank, syndicated letters of credit and/or letters of credit from a sole issuing bank in an aggregate amount of $400,000 and is available until September 2019, provided the Company is in compliance with all covenants. The 2014 Credit Facility has a sublimit for letters of credit issued thereunder of $50,000. The proceeds of these loans may be used for the Company’s commercial paper program or for general corporate purposes. The Company may increase the total amount available under the 2014 Credit Facility to $525,000 subject to certain conditions. No bank is obligated to provide commitments above their share of the $400,000 facility. The Company did not use the commercial paper program during the years ended December 31, 2015 and 2014 and there were no amounts relating to the commercial paper program outstanding at December 31, 2015 and 2014. The Company made no borrowings using the 2014 Credit Facility and no loans are outstanding at December 31, 2015. The 2014 Credit Facility contains restrictive covenants and requires that the Company maintain certain specified minimum ratios and thresholds. Among others, these covenants include maintaining a maximum debt to capitalization ratio and a minimum consolidated adjusted net worth. At December 31, 2015, the Company was in compliance with all covenants, minimum ratios and thresholds.

Debt

On March 28, 2013, the Company issued two series of senior notes with an aggregate principal amount of $700,000 (the “2013 Senior Notes”). The Company received net proceeds of $698,093 from this transaction, which represents the principal amount less the discount before offering expenses. The discount of $1,907 is being amortized over the life of the 2013 Senior Notes and is included as part of interest expense on the consolidated statements of operations. The first series is $350,000 in principal amount, bears interest at 2.50% per year and is payable in a single installment due March 15, 2018 and was issued at a 0.18% discount. The second series is $350,000 in principal amount, bears interest at 4.00% per year and is payable in a single installment due March 15, 2023 and was issued at a 0.37% discount. Interest on the 2013 Senior Notes is payable semi-annually on March 15 and September 15 of each year. The 2013 Senior Notes are unsecured obligations and rank equally with all of the Company’s other senior unsecured indebtedness. The Company may redeem each series of the 2013 Senior Notes in whole or in part at any time and from time to time before their maturity at the redemption price set forth in the Indenture. The 2013 Senior Notes are registered under the Securities Act of 1933, as amended.

The interest expense incurred related to the 2013 Senior Notes was $22,988, $22,981 and $17,357 for the year ended December 31, 2015, 2014 and 2013, respectively. There was $6,635 of accrued interest at both December 31, 2015 and 2014. The Company made interest payments on the 2013 Senior Notes of $11,375 on March 15, 2015 and 2014 and September 15, 2015 and 2014.

In February 2004, the Company issued two series of senior notes with an aggregate principal amount of $975,000 (the “2004 Senior Notes”). The Company received net proceeds of $971,537 from this transaction, which represents the principal amount less the discount before operating expenses. The discount of $3,463 is being amortized over the life of the 2004 Senior Notes and is included as part of interest expense on the statement of operations. The first series was $500,000 in principal amount, issued at a 0.11% discount, bore interest at 5.63% per year and was repaid on February 18, 2014. The second series is $475,000 in principal amount, bears interest at 6.75% per year and is payable in a single installment due February 15, 2034 and was issued at a 0.61% discount. Interest on the 2004 Senior Notes is payable semi-annually on February 15 and August 15 of each year. The 2004 Senior Notes are unsecured obligations and rank equally with all of the Company’s other senior unsecured indebtedness. The senior notes are not redeemable prior to maturity. All of the holders of the 2004 Senior Notes exchanged their notes in May 2004 for new notes registered under the Securities Act of 1933, as amended.

The interest expense incurred related to the 2004 Senior Notes was $32,127, $35,414, and $59,414 for the years ended December 31, 2015, 2014, and 2013, respectively. There was $12,023 of accrued interest at both December 31, 2015 and 2014. The Company made interest payments on the 2004 Senior Notes of $16,031 and $30,094 on February 15, 2015 and 2014, respectively, and $16,031 on August 15, 2015 and 2014.

Credit Facility

The Company’s commercial paper program requires the Company to maintain liquidity facilities either in an available amount equal to any outstanding notes from the commercial paper program or in an amount sufficient to maintain the ratings assigned to the notes issued from the commercial paper program. The Company’s subsidiaries do not maintain commercial paper or other borrowing facilities at their level. This program is currently backed up by a $400,000 senior revolving credit facility, of which $395,960 was available at December 31, 2015, due to $4,040 of outstanding letters of credit related to this program.

On September 16, 2014, the Company entered into a five-year unsecured $400,000 revolving credit agreement, as amended by Amendment No. 1, dated as of March 5, 2015 (the “2014 Credit Facility”) with a syndicate of banks arranged by JP Morgan Chase Bank, N.A. and Wells Fargo, N.A. The 2014 Credit Facility replaces the Company's prior four-year $350,000 revolving credit facility (the "2011 Credit Facility"), which was entered into on September 21, 2011 and was scheduled to expire in September 2015. The 2011 Credit Facility terminated upon the effectiveness of the 2014 Credit Facility. The 2014 Credit Facility provides for revolving loans and the issuance of multi-bank, syndicated letters of credit and/or letters of credit from a sole issuing bank in an aggregate amount of $400,000 and is available until September 2019, provided the Company is in compliance with all covenants. The 2014 Credit Facility has a sublimit for letters of credit issued thereunder of $50,000. The proceeds of these loans may be used for the Company’s commercial paper program or for general corporate purposes. The Company may increase the total amount available under the 2014 Credit Facility to $525,000 subject to certain conditions. No bank is obligated to provide commitments above their share of the $400,000 facility.

The Company did not use the commercial paper program during the years ended December 31, 2015 and 2014 and there were no amounts relating to the commercial paper program outstanding at December 31, 2015 and 2014. The Company made no borrowings using the 2014 Credit Facility and no loans are outstanding at December 31, 2015.

The 2014 Credit Facility contains restrictive covenants and requires that the Company maintain certain specified minimum ratios and thresholds. Among others, these covenants include maintaining a maximum debt to capitalization ratio and a minimum consolidated adjusted net worth. At December 31, 2015, the Company was in compliance with all covenants, minimum ratios and thresholds.