As of March 31, 2019 and December 31, 2018, the aggregate estimated fair value and carrying value of our global revolving credit facility, unsecured term loans, unsecured senior notes and mortgage loans were as follows (in thousands):



	Categorization under the fair value hierarchy	As of March 31, 2019				As of December 31, 2018
	Categorization under the fair value hierarchy	Estimated Fair Value		Carrying Value		Estimated Fair Value		Carrying Value
Global revolving credit facility ⁽¹⁾⁽⁵⁾	Level 2	$	857,211	$	857,211	$	1,663,156	$	1,663,156
Unsecured term loans ⁽²⁾⁽⁶⁾	Level 2	811,654		811,654		1,183,121		1,183,121
Unsecured senior notes ^(3)(4)(7)	Level 2	8,879,673		8,569,195		7,684,368		7,629,679
Secured debt ⁽³⁾⁽⁸⁾	Level 2	105,769		105,745		706,086		705,924
		$	10,654,307	$	10,343,805	$	11,236,731	$	11,181,880


(1)	The carrying value of our global revolving credit facilities approximate estimated fair value, due to the variability of interest rates and the stability of our credit ratings.


(2)	The carrying value of our unsecured term loans approximates estimated fair value, due to the variability of interest rates and the stability of our credit ratings.

(3)

Valuations for our unsecured senior notes and secured debt are determined based on the expected future payments discounted at risk-adjusted rates. The 2019 Notes, 3.400% 2020 Notes, 2021 Notes, 3.950% 2022 Notes, 3.625% 2022 Notes, 4.750% 2023 Notes, 2.750% 2023 Notes, 2.625% 2024 Notes, 2.750% 2024 Notes, 4.750% 2025 Notes, 4.250% 2025 Notes, 2026 Notes, 2027 Notes, 2028 Notes, 2029 Notes and 2030 Notes are valued based on quoted market prices.


(4)	The carrying value of the 3.400% 2020 Notes, 2021 Notes, 3.625% 2022 Notes, 3.950% 2022 Notes, 4.750% 2023 Notes, 2.750% 2023 Notes, 2.625% 2024 Notes, 2.750% 2024 Notes, 4.250% 2025 Notes, 2026 Notes, 2027 Notes, 2028 Notes, 2029 Notes and 2030 Notes are net of discount of $11.5 million and $19.9 million in the aggregate as of March 31, 2019 and December 31, 2018, respectively.


(5)	The estimated fair value and carrying value are exclusive of deferred financing costs of $14.2 million and $15.4 million as of March 31, 2019 and December 31, 2018, respectively.


(6)	The estimated fair value and carrying value are exclusive of deferred financing costs of $3.9 million and $4.2 million as of March 31, 2019 and December 31, 2018, respectively.


(7)	The estimated fair value and carrying value are exclusive of deferred financing costs of $45.7 million and $40.6 million as of March 31, 2019 and December 31, 2018, respectively.


(8)	The estimated fair value and carrying value are exclusive of deferred financing costs of $0.3 million and $20.2 million as of March 31, 2019 and December 31, 2018, respectively.